You should also try Sam Harris' The Moral Landscape. I don't agree with Sam's main point in the book, but it's a very engaging read. He has done something that no moral philosopher has done by coming close to establishing a basis for objective morality. I had to try very hard in order to refute a lot of his points and it proved to be a mental workout.
Interesting, it's on my list
BTW, what's your favourite Dawkins book?