The greatest place to find spell books is the deep, I bought the game on release and have been putting about 5 hours a day in, this morning I had found 2 spell books and 1 scroll book in total, after checking out the Deep area's (wierd places that are even more broken than the rest of this world) I ran out of spells to learn, found 12 spellbooks I think in one run, there is usualy 1 library or study, sometimes 2 in each house so a quick run through an area with petty bats and espers (I have been running rouund a level 32 deep area at level 25) and spells are no problem.
In fact I thing something needs to be done because its too easy to grab spell books from Deep area's, they look nasty, drop you into the middle of the map like the ocean, and have a mean descroption, but are by far the easyist region to loot houses in and have the best loot as well. Its practically broken the "have to search out rare spell books" aspect for me when I can peacefully stroll through a time shard where spell books were mass printed and posted through every door for free.