Lost Christianities by Bart D. Ehrman
I was a little bit afraid, on picking this book up, that it would be a New Age style thing, or that it would conflict with my beliefs (you have to be so careful these days!). As it turns out, it only affirmed my beliefs, and I learned a lot! A great book, with LOTS of history. The easy-to-read, semi-humorous nature of the book made it easy to like, even though the content required a bit of "brain power." :-)
Read the review of this book at FaithfulReader.com.