The Library

[Klengo]

The Library quest requires books that got, but sold as I didn't need them and I figured they wouldn't be needed for anything. Are there other ways to acquire these books after you first find them, or am I SOL?

[Lord-Xanthor]

Exit town, or just exit game and load back in and check with merchant. Unless things changed, doing the above causes what the merchants is selling to change as well

[derw4tz]

I had the same problem but remembered a dwarf merchant in the mine shaft; he actually sells the books.
I then checked other merchants and found the elve books in the starting town

TLDR:
Dwarf books can be bought at Hardsoil Bulwark (West of the Stonebridge City)
Elven books can be bought by Edbert in Woodbury (starting town)

(this is just for future players)