Excellent take on the theme. Great atmosphere and sound effects. I like the upgrade system, however the speed upgrade doesn't feel significant. Your attack speed always feels slow in proportion to the number of enemies. The "hidden" upgrade feels very powerful, but stopped working around level 6? You say in a comment there are 15 books, but I only ever found 3 different kinds. Sorry for nitpicking, this game is actually really good overall.
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Thank you very much for playing my game.
You mean the upgrade that makes the attack fly faster? I will look into that and patch it for the after-jam submission. But that the "book of the hidden" stopped working is strange. Maybe another upgrade overwrites that behavior, but that's not intentional. Do you remember your exact modifiers and their order? Then I will look into it.
That you only found three different books is really unlikely. While testing the game I have found nearly every book. Maybe you just have to play for a little bit longer. ;)
I hope you had a great time playing the game despite it's not full balanced yet.