I'm reading Lost Christianities by Bart Ehrman.
If you're interested in the Bible and Christianity it's a fascinating read. Marcionism was a Christian sect which could rival the sect which later became Catholisism in size. They branded each other as heretics. And if they'd won Christians today would have believed in two gods.
All the different variants of the Bible is amazing. The Gnostic Bible even survived.
http://en.wikipedia.org/wiki/Nag_Hammadi_library
Or the Ebionites who's gospel of Matthew has two extra chapters. Marcionism only took Paul's and Luke's seriously, and saw the rest as filler. Ebionites rejected Paul.
With all these different Bibles we get radically different Christian theories. It's an amazing read. Every page is mind-blowing. I read "the history of God" by Karen Armstrong. Which is a great book, but she describes a much simpler compilation of the Bible than what was actually the case. It's understandable, since it's long enough as it is.
I wouldn't call Lost Christianities my favourite book, but if you've got any interest in Christianity/theology, it should be a must read.