If you quick save before running your mouse over a merchant or hitting the alt key nearby, you can trade with him/her, then quick load to get a different inventory. After you have talked to an NPC, it takes around 15 minutes real time before their inventory will be reset, though this seems to vary a bit between NPCs.
Kistandalius or (sometimes) Blake in Ars Magicana and Corinna or (sometimes) Mpenzak in Verdistis are good when looking for spell books, though just about any NPC has a chance to have one.
