Try Disbelief 101: A Young Person's Guide to Atheism.
The Faith Instinct, by Nicholas Wade
Outstanding expose' into the biology of why humans have been so religious.
Infidel and Nomad, by Ayaan Hirsi Ali
One of the best current calls for reason from within the darkness of superstition. I recommend listening to the audio books, as her voice adds a whole dimension to her words.
Great Transformation, by Karen Armstrong
One of the greatest thinkers about the history of religion alive today, Karen's books (like "The History of God") are not written to promote nor dethrone religion, but to explain its history.