Nicomachean Ethics

There's a lot of fairly useful stuff in here, however, a lot of it's also fairly obvious (I hope). Book 2 was probably the most useful for me. The other books have very good explanations of concepts, but I didn't really need them so I quickly lost interest.

Over all this is a great into to philosophy but if you're slightly more seasoned (especial with the subject's Aristotle covers) it might be rather boring.