A good atheist book to give to a religious friend?

I have a good, somewhat Christian friend that I'd like to give a great book on atheistic arguments. I have read The God Delusion, A Letter to a Christian Nation, God is not Great, and The Portable Atheist. I don't really see these as an appropriate gift, I'm looking for a more scientific, formal, and philosophical book that will be intellectually stimulating for him. Thanks for the help!