What Makes Book of 99 Special?
Book of 99 by Relax Gaming stands apart from typical book-themed slots through its revolutionary 99% Return to Player rate and innovative collection mechanic. While most online slots hover around 94-96% RTP, this Greek mythology adventure returns nearly every penny wagered over the long term, making it one of the fairest games available to UK players.
The 5-reel, 3-row grid with 10 fixed paylines may seem standard, but the gameplay depth comes from the dual wild trigger system. The ornate golden Book symbol doesn't just substitute for other symbols—it's counted toward a persistent collection meter that tracks your progress across sessions.
Symbol Values and Payline Wins
The paytable showcases detailed Greek artwork. The premium Male Hero symbol delivers the highest regular payout at 1000x bet for five-of-a-kind, paying from just 2 symbols. The Female Prophetess offers 400x for a full line. Mid-tier symbols include the grotesque Cyclops and fanged Dark Sorceress, both paying 150x for five matches.
Low-paying symbols use the classic A-K-Q-J-10 card ranks, but stylised with 3D golden lettering and embedded gemstones in ruby, amber, amethyst, emerald, and sapphire. These pay between 20x and 30x for five-of-a-kind.
Paylines evaluate left to right starting from reel one, with only the highest win per line paid. The 10 paylines include three straight horizontal lines, zigzag patterns, V-shapes, and step patterns that create multiple ways to connect matching symbols.
Free Spins Expanding Symbol Feature
When you trigger free spins—either through landing 3+ Wild Books in a single spin or completing the 99 collection—10 spins are awarded. Before the feature begins, the game randomly selects one of the nine regular symbols as the special expanding symbol.
During each free spin, normal payline wins are evaluated and paid first. Then, if the special symbol appears in sufficient quantity (2 for premium character symbols, 3 for card symbols), those symbols expand to fill their entire reel. The expanded reels then pay for all 10 paylines based on the number of special symbols visible—even if they're scattered across non-adjacent reels.
This mechanic creates substantial win potential because a single spin with 3 or 4 expanded reels can pay as if you hit the same symbol combination across all 10 lines simultaneously. The Male Hero as expanding symbol with 5 visible delivers 10,000x your bet (1000x multiplier × 10 lines).
Strategy Considerations for UK Players
The 99% RTP applies across all bet levels, from the minimum stake through to the maximum. This means the collection mechanic works identically whether you're playing conservatively at £0.20 per spin or higher stakes at £100 per spin.
The weighted average bet calculation for collection-triggered free spins is crucial to understand: if you collect 50 wilds at £1 per spin and 49 wilds at £2 per spin, your free spins will play at approximately £1.50 per spin. The game uses stochastic rounding to ensure the bet stays at valid increments.
Turbo mode and autoplay are available for faster session play. The spacebar spin shortcut can be enabled in settings—particularly useful for extended sessions. Since the collection meter persists, you can safely close the game and return later without losing progress toward the 99-wild trigger.
Mobile Optimisation and Accessibility
The game runs flawlessly on iOS and Android devices through HTML5 technology. The interface scales perfectly to mobile screens, with the collection meter displayed as a vertical panel on the left side. Touch controls are responsive, with appropriately sized buttons for bet adjustment and spinning.
Relax Gaming has implemented comprehensive accessibility features including screen reader support, keyboard navigation, independent audio toggles, and colourblind-friendly visual indicators. The settings menu allows you to disable intro animations and splash screens for quicker access on repeat sessions.
Verdict: Is Book of 99 Worth Playing?
For UK players seeking maximum value, Book of 99's 99% RTP is genuinely industry-leading. The collection mechanic adds a persistence element that standard slots lack—you're always progressing toward a guaranteed free spins trigger, even during losing base game sessions.
The expanding symbol feature in free spins delivers the volatility spikes that experienced players seek, while the beautiful Greek mythology artwork and authentic ancient temple setting create an immersive atmosphere. This isn't a generic book slot reskin; it's a thoughtfully designed game where the mathematical model, features, and theme align perfectly.
Whether you're playing the free demo to understand the collection rhythm or wagering real money, Book of 99 offers transparency, fairness, and engaging gameplay that respects your bankroll more than almost any competing slot.