Just got word today that Richard Carrier has released his book, Not the Impossible Faith. It's essentially a rewrite of his online book, Was Christianity Too Improbable to Be False? And he tells me he has rewritten some sections in the book in order to provide greater clarity and made various other minor changes. He blogs on it here.
Anyway, the paperback copy looks pretty pricey (Nearly thirty dollars!) But the downloadable version is a steal at two dollars and fifty cents.
Just to avoid confusion: This book does not deal with Jesus Mythicism. Carrier's book on that won't be available for a good while I suspect. This book only deals with the claim that Christianity spread and survived because it was true.