By the rules, you cannot learn spells from a defeated enemies spellbook, just as a fighter can't learn new abilities from seeing enemies using them in battle, or a cleric can't learn new prayers by conversing with other priests of his faith.
Now, I do believe you can learn Rituals found in an enemy Ritual book, and add them to your own.
And, of course, a DM could certainly allow otherwise, and let you scribe new spells into your book from a defeated foe's spellbook, relying on the limitation of power use to mitigate the extra options you have when choosing prepared spells. I would recommend sticking with only letting Utility Powers and Daily Powers be learned from enemies, since those are already the ones the spellbook revolves around. While not entirely balanced to allow this, I suspect enemy spellbooks are rare enough that it will not be a major game changer, and is perfectly fine as a house rule.