One symbol does two jobs
Book of Dead is a five-reel Egyptian slot in the Rich Wilde series, and its whole structure hangs on the book symbol. The book acts as the scatter that opens the free-spin round and also substitutes as a wild in the base game, which means the symbol you are hoping to collect is useful even when it does not trigger anything.
The rest of the paytable is conventional: card ranks at the low end, then the themed symbols, with Rich Wilde and the higher-value characters at the top. Base-game spins are unremarkable on purpose, because the feature is where the game's returns are concentrated rather than spread across ordinary line wins.
The expanding symbol that defines the round
Triggering free spins runs an animation that selects one symbol at random and promotes it for the whole round. When that chosen symbol lands during the feature it expands to cover its entire reel and pays as though the positions were filled, ignoring the usual requirement that winning symbols sit on adjacent reels.
This single mechanic is why feature outcomes vary so widely. Drawing a high-value character gives the round genuine potential, while drawing a low card rank means most of the expansions pay very little. Additional books during the feature can extend it, and the promoted symbol carries through any extension unchanged.
Volatility, the gamble option and its siblings
This is a high-volatility slot with a thin base game, so sessions tend to be flat until the feature arrives, and plenty of features finish below the cost of reaching them. That profile is why it suits patient, small-stake play far better than short sessions with money that matters.
The game also offers the gamble option common to the studio's catalogue, which lets you risk a win for a larger one on a coin or card call. That is a variance choice with no strategic benefit. Play'n GO released the title in 2016, and the later Rich Wilde entries are built on similar foundations.