- You can get air and water spellbooks from a woman in the market square, standing next to a stall. - You can get ranger skillbooks from the fletcher, also market square. - You can get witchcraft books from a woman on the top floor of the inn. I forgot if she also sells scoundrel skillbooks. I think she does. - You can get fighter skillbooks from Aurius, in his headquarters.
As far as I know there's no one in Cyseal that sells fire and earth spellbooks. :S
Edit: Apparently Arhu sells fire/earth spells. Heh. Well, that's good to know.
Both the captain (go to the harbor, he's on the big ship to the south) and the Healer always sell a few odd books as well. I'm not sure about the quartermaster (she's upstairs above the cook in the mess hall) since I always forget she is there.