You can also use telekinesis to grab stuff out of walls. Spell books can be bought from Kistandalius or Blake in Ars Magicana and Corinna or Mpenzak in Verdistis. If you quick save before talking to a merchant, hitting alt or running your mouse over them, you can quick load if they don't have what you want. I'm not sure how long it would take looking for a specific spell book, though; I maxed this skill with spell books just keeping an eye open when selling off loot.
