They nerfed stairhopping, which was a technique where you could keep farming books between ID1 and ID2 without spending an in-game turn, because ascending/descending stairs apparently doesn't count as a turn. So you could spam < and > without ever getting hungry and there'd be a chance that a book spawns instantly inside.

As far as I understand, the Danger Level at which spellbooks are most likely to spawn is 1-2, so low ID levels should still be good for finding books if you are hardpressed for those.

That said, all I can say is counter with my own anecdote that my Priests always get a lot of spells. How far do you get before dying?