Well, I personally find it just easier to acquire any spell books that the wizards I encounter leave lying around (if wizards didn't want rogues to have their spellbooks then wizards would put better locks on them, be sure to check for magical traps since wizards like to kid rogues who borrow their spellbooks). It's all part of the standard rogue ethical structure, spellbooks contain knowledge and knowledge should be free to whatever rogue wants to use it.