I don't know if all bookcases can spawn spellbooks, but I've gotten random spellbooks from Landilor's House & Mardaneus's House in Aleroth right at the beginning of the game. It seems more common to find one at Mardaneus's house but reloads will give you both if you have time.
My warrior has gotten the spellshield spell which helped immensely in the catacombs vs. Skeletal Conjurers and the other big skeleton warriors, my wizard once got Elemental Hail from Mardaneus's and that made the catacombs and most orcs around Aleroth a complete joke.
