You know what? The more I read about this the more I think that the best choice would be to run around on Normal or Advanced difficulty, just to speed things up.
Black Kats appear (supposedly) with a 1% chance. Books (supposedly) drop with a 1% chance. Therefore being able to get the most Black Kats to spawn is the best choice....
So that's either rushing levels on Hard, or rushing levels on Normal... I guess it depends how much of the level is spent at parts that require you to kill enemies or get keys.
My general impression is that more loot drops in higher difficulty levels. I don't know if that increases the chance per se, but I would assume it does.