Thank you so much for your detailed review and for playing my game. :)
[SPOILER]
In total there are 15 books. Three of them are passive and will not effect the attacks. The result attack is also dependent on the order you found the modifiers and some modifiers can be found multiple times. So potentially there are more than 10^9 combinations. But to be fair, most of them will look alike.
[/SPOILER]
After the jam I try to provide a more performant version of the game. I think the issue is based on the many area2D nodes that the enemies have. The closer the submission date got, the more messy the code became. ;)
But nevertheless I am glad that you had a good time playing the game.