"The Jesus Mysteries" by Timothy Freke and Peter Gandy

Read it, didn't buy the theory. The authors didn't know the difference between Mithra (Persian deity) and Mithras (god of the Roman legions) to start with, and that's a pretty critical distinction. They also didn't talk enough about the story of Dionysus to get a real feel for any comparisons between Dionysus and Jesus, and there are some. Nonetheless, a "mystery religion" lens for viewing the Jesus narrative is useful for mining more significance out of it when the usual way of reading it runs dry. There are also some very nice colour photos.