The spell on the left edge of the screen are called enchantments, and they are supposed to help you in various ways to deal more damage to the demon. They are similar to jokers from balatro if you are familiar with that. They do different things like increase the number of repeats when you cast a spell or deal additional damage.
If you mean the "Spellbook X/Y" in the bottom left corner, this is just the counter of the "X damage"/"Y poison" spells (until you need to surrender and lose).