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.