Blake in Ars Magicana and (less often) Mpenzak in Verdistis also occasionally have spell books, though not as often as Kistandalius and Corinna.
If you quick save before running your mouse over an item/chest/merchant or hitting the alt key, you can identify the item or trade with the merchant, then quick load to get different stats or inventory. This can be done repeatedly to get the best stats or specific bonuses on an item (save after finding good candidates) or looking for specific items (such as spell books or charms) from merchants.
