i've just finished a playthrough, with about 27k. no cheating though I did box a lot..... lol
below is a small breakdown of the information sources on monsters + plants (SOME SPOILERS)
in Ch 1, old women can tell you about all the local plants(and some of the city plants. there's several of them, and they cycle, so you may have to approached them multiple times, lol. I've spent about half and hour trying to get description of Balissa) as well as the following monsters: Barghests, drowners, echinops, fleders. there's a book in one of the houses about the corpse-eaters. and, of course, you learn about the Beast from the notes.
in Act 2, you can learn about all the swamp plants by giving a Brickmaker woman honey. Seighfreid will tell you about the cocatrix. only book you need to buy is corpseaters pt II for the Alghoul contarct (at least only book I couldn't find). Fear and laothing II (the cursed creatures) is available in one of the houses. you may need the Golem book, but that fight's not about the fighting, if you know what I mean.
in Act 3, in New Naracort Inn, one of the regulars will tell you stories, if you get her milk or beer (2 jugs of milk in the Hairy bear Inn chest, if you're that stingy). she tells you about basilisks, Archespores, and Wyverns. not sure if it'd update the entry for Royal wyverns, b/c I'd read the book in Act 2, but it probably does. The only monsters I didn't find a source on were the Kikimoras. that's the one book you may need to buy. oh, and Vaska gives you the book on Dagon.
In Act 4, one old women will teach you all the local plants for sweets, 2 peasants will tell you about the noom/night shades and nearly half the villagers can't wait to tell you about the the last 3 vampire species. up to this point, only other vamps you'd have encountered are
1) the bruxa in "Echoes of the past"
2) the vamp family in the brothel.
Also, in the sidequests in Act 4, you have 3 books as optional quest rewards:
Nayad: the vodyanoi
Harn (mason) Ghosts+wild hunt
the Hermit: vampires
In Act 5, in Kalkstein's house there's a book of Alzur--breakdown of Koschey + the skullheads. I bought the Rigger book off the Druids in the cave, but it's optional. there's a Striga book, too.
In total, I think you need to buy 3 books all game, haha oh, and the Feainvedd one.
there's a lot of books about the world as well, but I don't think there's a single one that cannot be found instead of bought. you just need to search every house for it.