Tell me what the ‘Expanding Symbol’ really do in the bonus round?
At the start of the free spins, the game arbitrarily picks one regular symbol to be the special expanding symbol. If you get enough of this symbol on a payline during the bonus, it expands to cover the whole reel. This can create multiple winning combinations at once, bringing much bigger payouts. If the Book scatter itself gets picked, it acts as a wild expanding symbol, which is notably valuable.