What's the 'empty tome bug'?
In this version of the game the empty tome effect only kicks in after you pick a weapon upgrade. Meaning that if you're unlucky and have one or more weapons already on level 8 before you get the empty tome to full level, it's impossible for these weapons to profit from the cooldown reduction. So not getting tome up to level 5 as fast as possible/ before your first rank 8 weapon can lead to a permanent loss in damage