Quick Verdict
Book of 99 is a Greek-mythology-themed entry in the Book-of slot genre, developed by Relax Gaming and released in May 2021. Its defining data point is a developer-confirmed 99.00% theoretical RTP, substantially above the 95% to 96% industry norm, making it structurally attractive for players who prioritise long-run return over raw ceiling size. The main caveat is the max-win figure: Relax Gaming’s own press materials cite 5,000x stake as the win potential ceiling, while several third-party review sites list 12,075x, a discrepancy that remains unresolved and means the true ceiling is not independently verified from the developer’s product page alone. Volatility classification is also contested, with secondary sources ranging from medium to high, so session pacing should be treated as unpredictable. Against its closest catalogue peer, Relax Gaming’s Money Train 3 (which offers a vastly higher ceiling but a materially lower published RTP), Book of 99 trades ceiling for consistency of return, a trade-off that suits bankroll-aware players more than max-win hunters. Operator-by-operator RTP verification was not available for this review, so active variant confirmation at the in-game help panel is essential before depositing.
Slot at a Glance
| Field | Value |
|---|---|
| Provider | Relax Gaming (Gibraltar subsidiary UKGC-approved, 2021) |
| Release Date | May 2021 (confirmed in Relax Gaming press release) |
| Grid / Paylines | 5 reels, 3 rows, 10 fixed paylines |
| RTP (top) | 99.00% (developer-confirmed) |
| Volatility | Medium per Slot Gods secondary-source review; medium-high to high per other trackers; not rated on Relax Gaming’s product page |
| Max Win (developer-cited) | 5,000x stake (Relax Gaming press release) |
| Max Win (secondary-source) | 12,075x stake (Clash of Slots, SlotsTemple; not confirmed on developer page) |
| Max Win GB Cash Equivalent | Up to £25,000 at £5 GB cap (25+); up to £10,000 at £2 GB cap (18 to 24), both using developer-cited 5,000x figure |
| Developer Bet Range | Not confirmed on product page; GB online slots are capped at £5 per spin for players aged 25 and over, £2 for ages 18 to 24 |
| Bonus Features | 99 Book Symbol Collector, Free Spins with Expanding Symbol |
| Bonus Buy | Not available |
| Mobile | HTML5, available on iOS and Android |
RTP Versions Across UKGC Operators
Operator-configurable RTP is the single most underreported data point in slot reviews; the headline figure is often not the version running at the operator you will deposit at.
For Book of 99, operator-level RTP variant confirmation was not available during the research for this review. Relax Gaming’s press materials and product page state a single 99.00% RTP figure, and no secondary source has confirmed whether a reduced-RTP variant is available for operator deployment. Three UKGC-licensed operators are confirmed to carry the slot: Mega Casino, Lottoland, and PokerStars Casino. The active RTP setting at each of these operators could not be independently confirmed pre-login.
Because the 99.00% headline RTP appears to be a single published configuration rather than the top of a variant range, it may represent a fixed setting. However, Relax Gaming’s distribution model allows operators to configure their game feed, and it is not possible to rule out operator-side variation without access to each operator’s live help panel. Always confirm the active RTP in the in-game information panel after login before depositing real money.
Math Model
Book of 99 is built around a 99.00% theoretical RTP, developer-confirmed in Relax Gaming’s press release and product page, which positions it as one of the highest-published RTPs in the licensed online slot market. Volatility is contested across secondary sources, ranging from medium to high, and is not rated on the developer’s product page. The four subsections below cover the RTP detail, volatility in practice, the maximum win ceiling, and the hit frequency and bonus trigger rate.
Book of 99 RTP Detail
The 99.00% figure is developer-published, appearing in Relax Gaming’s May 2021 press release and product page, making it the highest-confidence source tier available. No secondary source has published a confirmed reduced variant, though operator configurability cannot be ruled out without pre-login panel access. The 99.00% RTP compares favourably against the 96.00% industry benchmark and against Relax Gaming’s own Money Train 3, which secondary sources list at around 96.00%. Demo builds served by third-party aggregators may reflect the provider’s default configuration, which under UKGC RTS 6 should match the corresponding real-money build on the same site, but may differ from any operator-side configuration.
Volatility in Practice
Relax Gaming’s product page does not display a named volatility rating for Book of 99. Secondary sources diverge: Slot Gods rates the game as medium volatility, while other trackers classify it as medium-high or high. The Book-of expanding-symbol mechanic concentrates most win value in the Free Spins round, which is a structural characteristic of high-volatility play even when the headline RTP is elevated. Players should plan session length around budget and time rather than anticipated bonus frequency, as gaps between feature triggers vary and the hit frequency is not published.
Book of 99 Maximum Win
Relax Gaming’s press release states win potential of up to 5,000x the bet in the Free Spins round. At the GB £5 stake cap for players aged 25 and over, this equates to a GB cash equivalent of up to £25,000. At the £2 GB cap for ages 18 to 24, the ceiling is £10,000 GB cash equivalent. A number of third-party review sites, including Clash of Slots and SlotsTemple, list a differing figure of 12,075x stake. This secondary-source figure is not confirmed on Relax Gaming’s own product page and should be treated as unverified from the developer’s documentation. The developer-cited 5,000x figure is the authoritative baseline; players should verify the current published ceiling in the operator’s game information panel before depositing.
Hit Frequency vs Bonus Trigger
Neither the base-game hit frequency nor the bonus trigger rate is published by Relax Gaming for Book of 99. Hit frequency is not disclosed on the product page or in available secondary-source databases. The bonus trigger rate, covering both the 3-scatter route and the 99-symbol collector route, is similarly unpublished. Without verified figures from either route, session pacing should be treated as unpredictable. Plan sessions on budget and time, not on expected bonus frequency.
Base Game
Book of 99 runs on a 5×3 grid with 10 fixed paylines, a standard layout for the Book-of genre. The Book symbol acts as both Wild and Scatter, substituting for all other symbols in the base game and triggering the Free Spins round when three or more land in a single spin. Symbol hierarchy follows the genre convention, with Greek mythology characters forming the high-value tier and card-rank symbols filling the lower tiers. Five-of-a-kind paytable values are not confirmed in independently verified sources for this review and are omitted to avoid citing unverified figures. The Book of 99 demo is available at most sites carrying the title, allowing inspection of paytable values before committing real money. Book of 99 runs on mobile via HTML5 on both iOS and Android, with no separate download required.
GB builds disable autoplay and turbo-spin per UKGC restrictions on rapid-play features, with a 2.5-second minimum game cycle under RTS 14D. These restrictions apply to the slot at any UKGC-licensed operator, not just specific sites, and affect session pace across all stake levels.
Book of 99 Bonus Features
Book of 99 concentrates bonus value into two routes to the same Free Spins round: the standard scatter trigger and a symbol-collection mechanic that functions as an alternative entry path.
99 Book Symbol Collector
The 99 Book Symbol Collector is the structural signature that differentiates Book of 99 from standard Book-of variants. During base-game play, each Book symbol that contributes to a win or appears on the reels adds to a running tally. Reaching 99 collected Book symbols triggers 10 Free Spins without requiring three scatters on a single spin. This mechanic gives players a second, accumulation-based route into the bonus round, which can meaningfully change session pacing for extended base-game runs. No multiplier applies within the collector mechanic itself; value is generated in the subsequent Free Spins round.
Book of 99 Free Spins with Expanding Symbol
Triggered by landing three or more Book symbols simultaneously on the reels, this round awards 10 free spins. Before spins begin, one of four Greek mythology hero characters is selected at random to become the expanding special symbol for the duration of the round. When the chosen symbol appears anywhere on a reel during a free spin, it expands to fill the entire reel, potentially covering all three rows and creating multi-reel expanded wins. Relax Gaming’s press release confirms this mechanic can produce win potential up to 5,000x the bet (developer-cited figure). Free spins cannot be retriggered within the round per available secondary-source information, though this is not confirmed on the product page. Against Book of Dead by Play’n GO, which uses the same expanding-symbol free spins structure at a lower 94.25% RTP (secondary source), Book of 99 offers a materially higher return within the same core mechanic.
Bonus Buy Status
No bonus buy feature exists in Book of 99. Reaching the Free Spins round requires either the standard 3-scatter trigger or accumulating 99 Book symbols via the collector mechanic. The bonus trigger rate for both routes is not published, so GB players should plan session length on budget and time rather than on any expected frequency to the bonus round. No UKGC-blocking consideration applies here, as no buy mechanic is present in the game file.
Cost of Play at Different Stakes
The published RTP is most useful when translated into theoretical cost per session. Below is the expected loss at four stake tiers assuming standard mobile pace of 600 spins per hour.
| Stake per Spin | Theoretical Hourly Loss (at 99.00% RTP) |
|---|---|
| £0.10 | £0.60 |
| £1.00 | £6.00 |
| £2.00 (GB cap for 18 to 24) | £12.00 |
| £5.00 (GB cap for 25+) | £30.00 |
Worked example: at 99.00% RTP and £1 stake, theoretical hourly loss = 600 x £1 x (1 minus 0.9900) = £6.00. At £5, the figure rises to £30.00. No reduced RTP variant is confirmed, so a second column is not applicable here; if an operator-side variant exists that cannot be pre-login verified, the true cost per hour at that variant could be materially higher.
These figures are statistical averages across many hours, not session predictions. Medium-high-volatility or high-volatility slots produce sessions far above or below these figures session-to-session. The numbers show the long-run cost the operator’s edge produces at the published RTP setting. Because operator-side variant confirmation was not possible for this review, the active configuration at your chosen operator (see the RTP Versions section above) is the most important figure to verify before depositing. Even at a reduced 97.00% RTP, hourly theoretical loss at £1 stake rises from £6.00 to £18.00, a threefold increase that a pre-login panel check can reveal.
Demo vs Real Money
Under UKGC RTS 6, a play-for-free game offered on the same site as the real-money version should accurately represent the corresponding play-for-money configuration. Third-party demo aggregators may serve the provider’s default demo build, which is not always the same RTP version selected by a UKGC-licensed operator for real-money play. The figure that applies to a deposited session is the RTP shown in the real-money in-game information panel after login, not the RTP displayed in a third-party demo environment.
The practical approach is to use the Book of 99 demo to test the expanding symbol mechanic, observe how the 99-symbol collector progresses during extended base-game play, and assess whether the bonus frequency suits your session tolerance, then confirm the active RTP at the operator before committing real money. The gap between third-party demo RTP and operator real-money RTP is most significant on multi-variant configurations, which Book of 99 may carry at some operators despite a single published figure.
The Book of 99 free demo is available without registration at most sites carrying the title, giving a meaningful opportunity to evaluate the bonus mechanics and symbol-collector route before deciding whether to deposit.
Pros and Cons
| Confirmed Strengths | Confirmed Weaknesses |
|---|---|
| 99.00% RTP is developer-confirmed, among the highest in the licensed slot market | Max-win figure disputed: 5,000x (developer) vs 12,075x (secondary sources), with no resolution from the product page |
| 99-symbol collector mechanic adds a second, accumulation-based route to Free Spins not present in standard Book-of titles | Volatility classification is inconsistent across secondary sources, making session pacing difficult to anticipate |
| Greek mythology theme with four distinct expanding hero characters adds minor visual variety to the free spins round | 5,000x developer-cited max-win ceiling is modest against high-volatility peers such as Money Train 3, which secondary sources list at 100,000x |
| HTML5 mobile compatibility across iOS and Android, no download required | Hit frequency and bonus trigger rate are both unpublished, preventing objective session-length planning |
| No bonus buy needed for GB players; the collector mechanic provides a second entry route without a premium cost | Operator-level active RTP variants cannot be pre-login verified, introducing uncertainty despite the headline 99.00% figure |
| Distributed to multiple UKGC-licensed operators including PokerStars Casino, Mega Casino, and Lottoland | Free spins retrigger is not confirmed in available sources, limiting potential spin count in the bonus round |
Who This Slot Suits
The central question for Book of 99 is whether the 99.00% theoretical RTP offsets the modest developer-cited max-win ceiling and contested volatility profile.
Bankroll-aware session players
Book of 99 is structurally well-suited to players who prioritise long-run return over ceiling size. The 99.00% RTP reduces theoretical hourly loss to £6.00 at £1 per spin at standard pace, lower than most comparably structured slots at the 95% to 96% RTP band. The catch is that the bonus-heavy structure means long base-game stretches can occur between significant returns, so a session budget that accommodates variance is still necessary even at an elevated RTP.
Bonus-mechanic enthusiasts
The expanding-symbol free spins round is a genre-standard mechanic rather than a structurally distinctive one, but the 99-symbol collector adds a layer of base-game engagement that Book of Dead, Play’n GO’s genre-defining title at a secondary-source 94.25% RTP, does not offer. Players who find the standard 3-scatter trigger too passive may find the collector mechanic extends engagement meaningfully.
Casual or short-session players
The Book-of expanding-symbol structure does not support short sessions reliably, as base-game hit frequency is unpublished and most value is concentrated in the bonus round. Players seeking consistent base-game return should consider a lower-volatility alternative. Is Book of 99 worth playing for casual players? Only if the 99.00% RTP is the primary motivation and session length can accommodate potential dry spells between features.
Players seeking a max-win ceiling above 10,000x, or a feature-rich mechanic beyond the expanding-symbol genre template, should look elsewhere. Money Train 3, Relax Gaming’s own high-ceiling flagship, is structurally closer to that goal despite its lower published RTP.
Alternatives by Player Goal
If you want a higher published RTP
Book of 99 at 99.00% already sits at the upper end of the published RTP range. Very few licensed online slots publish a higher verified figure. If this title is not available at your operator or the active variant is lower than the headline, there is no directly verified higher-RTP Book-of alternative in the current Relax Gaming catalogue.
If you want a higher ceiling
Money Train 3, Relax Gaming’s own high-volatility cash-collect title, has a secondary-source max-win listing of 100,000x stake, dwarfing Book of 99’s developer-cited 5,000x. The trade-off is a substantially lower published RTP. For players whose primary goal is ceiling size, Money Train 3 serves that goal at the cost of return rate.
If you want lower volatility
Book of 99’s volatility is contested at medium to high. For a confirmed lower-volatility experience within the Book-of genre, Book of Dead by Play’n GO is rated as high volatility by its developer, which makes a direct lower-volatility comparison difficult within the same mechanic. Outside the genre, NetEnt’s Starburst, a classic expanding-wild title, carries a secondary-source volatility classification of low to medium and a published RTP in the 96% range, making it a structural lower-volatility alternative if the genre constraint is relaxed.
If you want the same theme
Age of Gods by Playtech uses the same Greek mythology setting with a progressive jackpot overlay. The RTP and mechanics differ substantially, but players drawn to Greek mythology characters as the visual theme will find thematic overlap.
If you want the same core mechanic
Book of Dead, Play’n GO’s genre-defining expanding-symbol free spins title, shares the same structural mechanic at a lower secondary-source RTP of approximately 94.25% and a 5,000x max-win ceiling. The 99-symbol collector in Book of 99 is the primary differentiator for players already familiar with the Book of Dead bonus structure.
Similar Slots
The following verified alternatives share structural similarities with Book of 99 across the Book-of mechanic, Greek mythology theme, or expanding-symbol free spins format.
| Slot | Provider | RTP | Max Win | Volatility | Key Difference |
|---|---|---|---|---|---|
| Book of Dead | Play’n GO | ~94.25% (secondary source) | 5,000x | High | Genre originator; no symbol-collector mechanic; lower RTP |
| Rise of Olympus | Play’n GO | ~96.00% (secondary source) | 5,000x | High | Greek mythology theme; Power of Gods mechanic rather than expanding symbol |
| Money Train 3 | Relax Gaming | ~96.00% (secondary source) | 100,000x (secondary source) | Very High | Same provider; vastly higher ceiling; cash-collect mechanic not Book-of |
| Scroll of Dead | Play’n GO | ~96.00% (secondary source) | 5,000x | High | Egyptian variant of same expanding-symbol structure; lower RTP |
Note: RTP figures marked with a tilde reflect secondary-source database data not independently confirmed from each developer’s own product page during this review.
Book of Dead, Play’n GO’s genre-defining title, is the most direct structural comparator for players already in the Book-of mechanic space. Its core free spins expanding-symbol round is functionally identical to Book of 99, but the approximately 94.25% secondary-source RTP is a meaningful step below Book of 99’s developer-confirmed 99.00%. Players for whom RTP is the primary variable, rather than theme or brand recognition, will find Book of 99 the stronger choice on the math model alone.
Where Book of 99 Sits in Relax Gaming’s Catalogue
Relax Gaming operates as both a developer and a B2B aggregation platform, with a catalogue that spans low-to-medium return titles through to high-ceiling, high-volatility flagships.
| Slot | RTP | Max Win | Volatility | Position vs Book of 99 |
|---|---|---|---|---|
| Book of 99 | 99.00% (dev-confirmed) | 5,000x (dev-cited) | Medium to High (secondary source) | — |
| Money Train 3 | ~96.00% (secondary source) | ~100,000x (secondary source) | Very High (secondary source) | Far higher ceiling; lower RTP; cash-collect mechanic |
| Temple Tumble 2 Dream Drop | ~96.00% (secondary source) | ~50,000x (secondary source) | High (secondary source) | Higher ceiling; Dream Drop jackpot overlay; lower RTP |
| Joker Troupe | ~96.00% (secondary source) | ~5,000x (secondary source) | Low to Medium (secondary source) | Lower volatility profile; similar ceiling; lower RTP |
Note: comparator figures for Money Train 3, Temple Tumble 2 Dream Drop, and Joker Troupe are drawn from secondary-source databases and are not individually confirmed from each title’s own developer product page.
Within Relax Gaming’s catalogue, Book of 99 occupies a distinct niche: the highest-published-RTP release, positioned against titles that offer far larger ceilings at materially lower return rates. On RTP alone it sits at the top of what is publicly available from the studio. On max-win ceiling, the developer-cited 5,000x is modest relative to the studio’s own high-volatility flagships. This is Relax Gaming’s high-RTP, genre-mechanic release in the Book-of space, differentiated by return rate rather than ceiling size or feature complexity.
How This Review Was Verified
Sources verified May 2024 to May 2025. The Relax Gaming product page at relax-gaming.com/products/casino/bookof99 and the May 2021 press release were the primary developer sources consulted for RTP, max-win, and feature details. The UKGC public register was checked for Relax Gaming (Gibraltar) Limited (account 57758) and Relax Gaming Limited (account 37462). Secondary databases consulted include Slot Gods, Clash of Slots, SlotsTemple, and AskGamblers for volatility classification, alternative max-win figures, and player sentiment. Operator availability was confirmed at Mega Casino, Lottoland, and PokerStars Casino via public game lobby pages; active RTP settings at each operator could not be confirmed without pre-login panel access and are therefore not stated in this review. The max-win discrepancy (5,000x developer-cited vs 12,075x secondary-source) was found but not resolved; both figures are presented with their respective source tags. Volatility classification is noted as contested. Hit frequency and bonus trigger rate were not found in any source. Players should treat any figure here as point-in-time and confirm the active configuration in the operator help panel before depositing real money. RTP, volatility, stake limits, and feature availability are subject to operator-side updates this review cannot pre-empt.
Responsible Gambling
Book of 99’s contested volatility profile, spanning medium to high across secondary sources, means that even at a 99.00% theoretical RTP, base-game returns can fall well below the headline figure across hundreds of spins. The expanding-symbol Free Spins round concentrates most win value in a single feature, and the 99-symbol collector route requires sustained base-game play before triggering, creating potential for extended dry stretches between meaningful returns. Set a session budget before play, set a time limit, and use the deposit and session limits available at your operator. Do not treat the high RTP as a short-term return guarantee.
Expected theoretical depletion can be estimated as: bankroll divided by (stake multiplied by (1 minus RTP)). For a £100 bankroll at the £5 GB cap and 99.00% RTP, this equates to approximately 2,000 spins to theoretical depletion at expected value, though variance can shorten or lengthen this dramatically in any real session. If you are concerned about your gambling, GamStop provides free self-exclusion across all UKGC-licensed operators with a single registration covering 6-month, 1-year, or 5-year periods. GamCare operates a 24-hour support helpline on 0808 8020 133. The UK Gambling Commission publishes guidance on player protections available at licensed operators.
Frequently Asked Questions
Relax Gaming’s own product page and press release confirm a 99.00% theoretical RTP for Book of 99, making it one of the highest published RTPs in the licensed online slot market. This figure is a long-run statistical average across millions of spins, not a session guarantee. Operator-level RTP variants are not confirmed in available sources, so always check the in-game help panel after login to verify the active configuration at your chosen casino.
Relax Gaming’s own press release states win potential rises to 5,000x the bet in the Free Spins round. Several third-party review sites list a differing figure of 12,075x stake, but this is unconfirmed on Relax Gaming’s product page and should be treated as a secondary-source or community-tracked figure only. At the GB £5 stake cap for players aged 25 and over, the developer-cited 5,000x ceiling equals a GB cash equivalent of up to £25,000.
No bonus buy feature is listed on Relax Gaming’s product page or press materials for Book of 99. The standard route into the Free Spins round is via three or more scatter symbols landing in a single spin. A second route exists through the 99 Book Symbol Collector mechanic, which requires accumulating 99 book symbols during base-game play. UK players have no bonus-buy shortcut available for this title.
Yes. Book of 99 is distributed by Relax Gaming, whose Gibraltar subsidiary holds approval from the UK Gambling Commission, enabling supply to UKGC-licensed operators. The slot appears at multiple UKGC-licensed sites including Mega Casino, Lottoland, and PokerStars Casino. Each of these is a separately licensed operator; availability and active RTP settings may vary between them. Check the operator’s game lobby and in-game information panel to confirm current configuration.
A Book of 99 demo is available at most sites carrying the title, allowing you to test the scatter trigger, the expanding symbol mechanic, and the 99-symbol collector without depositing. Under UKGC RTS 6, a play-for-free build on the same site as the real-money version should reflect the corresponding real-money configuration, but third-party demo aggregators may serve the provider’s default build, which can differ from the operator’s active RTP variant.
Volatility classification for Book of 99 is not consistent across sources. Slot Gods rates it as medium volatility. Other secondary-source trackers lean toward medium-high or high volatility. Relax Gaming’s own product page does not display a named volatility tier. The 99% RTP reduces the long-run house edge substantially, but the Book-of expanding-symbol mechanic concentrates most value in the Free Spins round, meaning base-game returns can be modest and gaps between significant wins vary unpredictably.
