Antiquary in Temple Quarter, Bookseller (he's in the marketplace, can't be more specific as he runs around if it rains) in Trade Quarter, one of the Druids in their Grove, and Kalkstein all have at least a few books.
The books aren't random, but I never paid much attention to where I got them. If you can kill a creature in a chapter, then the book for the creature should be available from one of the book sellers in the chapter.
SWC
Sir Edmund:"Should you obey the lord who asks you to put a village of innocents to the torch? Is that chivalrous? Is it noble?" Me:"It's a great way to get promoted, I know that much."
Checked in Act 1 (And his inventory didn't noticibly change the last time I played between the acts), the Dwarf Antiquary in the Temple Quarter (near Shani's House) has a copy for 500 Oren.