Book of 99 Slot Review: The Complete Relax Gaming 99 Percent Guide

US availability note, read this first. Book of 99 is made by Relax Gaming, and Relax Gaming only entered the regulated US market in the mid-2020s, launching first through BetMGM in New Jersey under the name RLX Gaming. As of this writing, Relax Gaming’s US rollout has begun with other titles from its catalog, and Book of 99 itself had not yet been confirmed in a legal US online lobby. That is an important distinction on a page about the regulated market: the studio is now live in the United States, but this specific game may not be loadable in your state yet. Treat the availability details below as the current picture and always check the live lobby, because a game that is not in a US casino today can appear the moment the operator adds it to its Relax catalog.

Everything that follows is built by Relax Gaming, and Book of 99 is a video slot wrapped in an ancient Greek theme drawn from The Odyssey, with Odysseus, a sorceress, a siren and a one-eyed Cyclops standing in for the usual pharaohs and explorers of the book-slot genre. What makes it worth a full page despite being a single title with no sequels is the 99 percent return, a figure so unusual that it is the entire reason the game exists and the first thing any value-minded player should understand before spinning. Our RTP explainer is a useful companion for exactly why that number matters.

Quick-view table

These headline numbers describe Book of 99 as Relax Gaming ships it. Where a value is not published by the studio, it is marked that way rather than guessed.

Version comparison table

Book of 99 is a genuine single title. There is no Book of 99 2, no land-based cabinet, no bonus-buy edition and, unusually for a book slot, no menu of alternate RTP builds. Relax Gaming publishes the game in one configuration only, the 99 percent return that gives it its name, which is the opposite of how a title like Book of Dead ships. The table below therefore captures the one build that exists and confirms, plainly, that there are no major spin-offs to compare it against. If a lobby ever lists a differently named Book of 99 machine, the safe assumption is a re-skin or a demo build, not a new game, and the specification will still read as below.

There is no land-based Book of 99 machine and no reduced-return version in circulation. Every instance of the game described on this page is the same online HTML5 slot running the same 99 percent math. That single-configuration approach is deliberate: the whole marketing pitch of Book of 99 is that it always returns 99 percent, so shipping a quietly lowered build would defeat the point.

What Book of 99 is and how it plays

Relax Gaming released Book of 99 on May 4, 2021, and pitched it directly at players who chase return to player figures. The setup is deliberately familiar to anyone who has spun a book slot before. You get a plain 5-reel, 3-row grid with 10 fixed paylines, a stack of themed picture symbols above the usual card ranks, and one special book symbol that triggers the free spins. What is not familiar is the return: at 99 percent, Book of 99 leaves a house edge of just 1 percent, roughly a third of the edge on a typical 96 percent to 97 percent slot, which is the single reason the game has a following that outsizes its otherwise ordinary feature set.

The theme leans on Homer’s Odyssey rather than the well-worn Egyptian tomb of most book games. The cartoonish, sun-drenched art puts a hero modeled on Odysseus at the top of the pay table, backed by a hooded sorceress in the mold of Circe, a winged siren and a hulking Cyclops standing in for Polyphemus. It is a lighter, more comic look than the grim Egyptian slots it competes with, and the soundtrack matches, keeping the tone playful even during the long dry spells that its high volatility guarantees. A new player understands the base game in a single spin, which is exactly the point of a book slot.

Do not let the friendly art or the generous RTP fool you about the risk. Book of 99 is rated a maximum 5 out of 5 for volatility on Relax Gaming’s own scale, with a hit frequency of just 29.83 percent, meaning fewer than one in three spins returns anything at all. The 99 percent figure is a long-run average measured across an enormous number of spins, not a promise about your session, and independent long-session tests have recorded actual returns well below the advertised number over short samples, which is exactly how high variance behaves. If you want to size a session sensibly, read our guide to volatility first, because this game punishes an oversized bet faster than its gentle looks suggest. To see how the regulated market is laid out, our real money slots hub lists operators by state.

Symbols

The pay table is short and, because there is only one build, entirely consistent. Symbols split into premium Greek characters, low-paying card ranks, and the single book symbol that does the work of both wild and scatter. Values below are expressed as multiples of the total stake for a five-of-a-kind win where published.

High-pay symbols

• The hero, an Odysseus figure: the top standard symbol, paying up to 500 times the stake for five on a line, the largest regular payout in the game and the symbol players most hope to see chosen as the expander.
• The sorceress, a hooded Circe-style enchantress: the second premium, a strong pay in its own right.
• The siren: a winged mid-premium Greek monster that fills out the upper pay table.
• The Cyclops, a one-eyed Polyphemus: the fourth character symbol, rounding out the premiums above the card ranks.

The four character symbols are the ones worth chasing in the bonus, because they need only two on a line to start paying, unlike the lower ranks. When the game selects a character as the round’s expanding symbol, the ceiling of the bonus rises sharply.

Low-pay symbols

• Ace, King, Queen, Jack and Ten, each dressed up with a gem to match the theme. These are the filler symbols that keep the base game ticking over and need three or more on a line to pay, with the smallest returns on the board.

Wild and scatter

• The Book of 99: the only special symbol, and it does two jobs at once. As a wild it substitutes for every paying symbol to complete a line. As a scatter it triggers the free spins when three or more land in view, though unlike some book games it awards no separate scatter cash payment for landing them. Every book you land also advances the collection meter described below, which is the feature that truly separates this slot from its rivals.

Bonus-symbol cross-check

Because players sometimes assume a game marketed on a special number must have several bonus symbols or several builds, here is the consistency check this page is required to make. Book of 99 has exactly one bonus symbol, the Book of 99 itself, and there is exactly one version of the game. The same book acts as wild and scatter, triggers the free spins, and feeds the collection meter, and it behaves identically whether you reach the bonus by landing three scatters or by filling the meter to 99. Nothing about the trigger symbol changes across any configuration, because there is only one configuration. So the answer to “do all versions share the same bonus symbol” is a clean yes: one game, one book, one 99 percent build.

How to trigger the bonus

Book of 99 is unusual because it gives you two separate roads into the free spins, and knowing both changes how you read a session. The first is the classic book-slot trigger: land three or more Book of 99 scatters anywhere on the reels in a single spin and you open 10 free spins immediately. The book does not need to sit on a payline, only on the screen, and three, four or five scatters all award the same 10 spins. There is no bonus buy on the game, so this trigger, and the one below, are the only ways in.

The second road is the wild collection meter, and it is the mechanic that makes Book of 99 more than a re-skinned book slot. Every single book symbol that lands during the base game, even a lone one that does nothing on the reels, advances a meter on the side of the screen by one step. When that meter reaches 99 collected books, the game automatically awards a set of 10 free spins on top of anything else happening. Because a book lands far more often as a single symbol than as a trigger-worthy trio, the meter is a slow, grinding second path to the bonus that rewards long sessions. One important detail: free spins won from the meter are played at the average bet size you used while filling it, not necessarily your current bet, so the payout scale reflects the whole run rather than a single stake. If you are new to the difference between a scatter trigger and a line win, our how to play guide walks through it.

How the bonus works

This is the section that defines the game, and it works the way the classic book-slot bonus does with the studio’s own twist. The moment the free spins begin, whether triggered by three scatters or by the collection meter, the book opens to a random page and selects one paying symbol to become the special expanding symbol for the entire round. It can be any symbol, from the low-paying Ten all the way up to the Odysseus hero, and that choice is locked for all 10 spins. It is the single biggest factor in how much the bonus pays, exactly as it is in the book slots that inspired it.

During the free spins the game plays like the base game with one powerful addition. Whenever the chosen special symbol appears, and there are enough of it to matter, it expands to cover every position on each reel it lands on, then pays as though it filled all three rows of those reels, even when the copies are not adjacent in the traditional payline sense. A character symbol needs just two to expand and pay, while a card rank needs three, which is why a bonus that picks the hero or the sorceress is worth far more than one that picks a Ten. A full screen of the expanding symbol can pay up to 5,000 times the stake in a single free spin, and stacking big expanded spins across a round, with retriggers, is how the game reaches its published 12,075x ceiling.

The round is retriggerable throughout. Land three or more books again during the free spins and you win another 10 spins on top, keeping the same chosen expanding symbol, so a single lucky bonus can run well past the base 10 games. There is no wheel, no pick screen and no second bonus beyond the collection meter feeding you into these same free spins. The entire personality of Book of 99 lives in that one expanding-symbol round, sharpened by the extreme volatility that makes the good rounds rare and the great ones rarer still. Our random number generator explainer covers how certified software keeps that math honest.

Version by version

Book of 99 is a single title, so this section reads differently from a multi-game franchise page. Rather than a string of sequels, what a US player needs to understand is the one build that exists, the fact that there are no reduced-RTP variants, and the honest state of its availability. Each subsection below covers one of those points in full.

Book of 99, the 99 percent build (2021)

This is the game, and effectively the only version of it. Released May 4, 2021, it runs 5 reels, 3 rows and 10 fixed paylines, returns 99 percent to the player for a house edge of just 1 percent, and carries a maximum win of 12,075 times the stake. The Book of 99 is the combined wild and scatter, three of them award 10 free spins with one random expanding symbol, and the wild collection meter delivers a second 10-spin bonus once 99 books have been gathered in the base game. Volatility is a maximum 5 out of 5, the hit frequency is 29.83 percent, and betting runs from 0.10 to 100 per spin depending on the operator. This is the configuration every honest lobby should be loading, because the 99 percent return is the entire reason the game was built, and there is no lower build hiding behind the same art.

What that means in practice is that Book of 99 is a specialist’s slot rather than a casual one. The tiny house edge makes it genuinely attractive to players who grind long sessions and care about squeezing every fraction of a percent of return, but the same design that gives it that edge, an extreme-variance model with a sub-30 percent hit rate, makes it brutal on a small bankroll and slow to reward. Two players can spin the identical 99 percent build and have wildly different sessions, not because the math differs but because variance this high produces enormous swings around the average. The number in the information panel will read 99 percent every time; how your night goes is a separate question entirely.

Alternate RTP builds

There are none, and that is worth stating plainly on a book-slot page where alternate builds are the norm. Most book games, Book of Dead being the classic example, ship in a spread of RTP configurations from the high 90s down into the 80s, and the operator chooses which to load. Book of 99 does not work that way. Relax Gaming publishes it in a single 99 percent configuration, and the studio has treated that as fixed, because a lowered build would contradict the one thing the game is famous for. If Relax ever releases a reduced-return version, it would be a departure from how the title has been sold, and you would confirm it the same way you confirm any slot: by reading the RTP in the game’s information screen before you deposit. Today, that screen should read 99 percent everywhere the game is offered.

Sequels, spin-offs and land-based ports

Book of 99 has no direct sequel, no themed spin-off and no land-based cabinet. It exists only as an online HTML5 slot, built in the browser for desktop and mobile, and it has never been manufactured as a physical machine for casino floors. That keeps the availability picture simple: if you are playing Book of 99, you are playing this one online game. The only variant you will encounter is the free demo, which runs the identical math in play-money mode and is a sensible way to feel out the volatility before committing real funds. Any other Relax Gaming book-style title you see is a different game, not a Book of 99 edition.

Max win and RTP in context

The maximum win on Book of 99 is 12,075 times the total stake. That figure is worth explaining, because reviews frequently confuse it. The 5,000x number that appears on many pages is the most a single free spin can pay when the screen fills with the expanding symbol; it is a per-spin cap, not the game’s ceiling. The true maximum, reached by chaining big expanded spins and retriggers across a bonus round, is the 12,075x published by the studio. It is a rare outcome on any high-volatility slot, and rarer still on one with a hit frequency under 30 percent, but it is a genuine single-run possibility rather than a theoretical stack of unrelated features.

The return figure is where Book of 99 stands almost alone. At 99 percent it sits among the highest-paying slots ever released to the mainstream market, roughly a third of the house edge of an ordinary 96 percent to 97 percent game. That does not make it a soft touch. A low house edge spread across extreme variance still means long losing stretches, and the 99 percent is an average that only expresses itself over a very large number of spins. The practical takeaway is the one this whole genre teaches: the number in the information panel tells you the long-run math, and on this game that number is exceptionally good, but variance decides your actual session. For a broader primer on whether certified lobbies are trustworthy, our safety guide is a useful read.

Mobile play

Book of 99 was built in HTML5 and runs in the browser on iOS and Android with no separate download. The 10-line grid scales cleanly to a portrait phone screen, the expanding-symbol free spins and the side-mounted collection meter both render clearly on a handset, and touch controls cover bet selection, spin and autoplay. Because the game is a plain 5-reel layout rather than a sprawling megaways or cluster grid, it is one of the more comfortable slots to play one-handed, and the collection meter in particular suits mobile, where longer casual sessions are common and the slow book count keeps ticking upward. For a broader look at how these titles handle small screens, see our mobile slots guide.

Where to play

Because Relax Gaming’s US distribution is still expanding, the fastest way to find Book of 99 in the legal market is to start from the regulated operators serving your state rather than the game itself. Our legal sites directory lists the licensed real-money casinos by jurisdiction, so you can see which lobbies carry Relax Gaming and, once inside, search for the title and confirm the posted 99 percent RTP before you deposit. Availability shifts as operators add studios and titles, so the in-lobby search box is always the final word on whether this specific game is live where you are.

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