How about Carl Sagan's The Demon-Haunted World ? IMO it is written better than The God Delusion, and it's more fun to read.

Julia Sweeny's Letting Go of God was also very entertaining, though perhaps it's best to get it in audio format. She reads it extremely well.

On the other hand, I'd can not recommend using Sam Harris's The End of Faith. It just reads like an angry Internet rant... I'm kind of surprised that the book is so popular.

And if you're looking for something subversive, there's always fiction ! C.S.Lewis isn't the only one with tricks up his sleeve. I'd recommend Roger Zelazny's Lord of Light, which is one of my favorite books of all time.