The game has three save slots. Each one is a separate run with its own hero.

To switch, open Gear, tap the ⚙ cog, and choose a slot under Saves. Switching saves what you are doing first, so you can keep a Warrior and a Mage going at once. Each slot shows its hero's class and level, and how many books they have finished.

Each hero's books are their own. The fourth book appears for a hero who has finished the other three themselves; books finished in another slot don't count towards it.