I think that the sort stack is a bit underpowered, since usually there aren't many of the same book in one stack anyway with how many stacks there are, and you should maybe get an additional sorted stack for every time you buy that upgrade (ie sort leftmost stack the first time, then the second and third stacks, etc)
Also, it would be nice to be able to drop books in your inventory back into the stacks, since sometimes you just click on the wrong book and don't want to find where it goes.
A bot that auto sorts stacks could be cool too.