I've seen a lot of those books myself, actually.
I got most of them from Navery, but I also got them from the following creatures (number received/number killed):
Primeval Lich 1/1
Slasher of Veils 3/3
Navery Night Eyes (about 40-50%, lost count at 5/10)
Wyvern [reknowned] 1/4
Goblin Slave 1/many
Ribbon clan ratman 1/many
As you can tell, I prefer Ms. Nighteyes because she's easy to kill (mage + EV + arachnid slayer + ebolts = dead spider fast), so the books can be collected more quickly. My goal is to assemble a complete collection of quest-reward books before October.