I actually read this book in a Swedish translation, probably during my high school days. Originally a German work, it argues that Jesus (whom the author insists on calling Rabbi J) had been a member of the Essene monastic community at Qumran, together with John the Baptist. Jesus and his disciples were probably Zealots, calling for some kind of Messianic action against Rome. Why else would the Romans have executed Rabbi J by crucifixion, the punishment reserved for political rebels?
The author, a journalist and rogue theology student, believes that a comparison between the Gospels and the Dead Sea Scrolls shows that whoever wrote the former were under Essene influence. Paul was the real founder of Christianity as we know it, and the Pauline influence explains why the anti-Roman and Qumranite tendencies in the Gospels are played down, although they are still visible to the discerning eye. It's not clear whether Lehmann is an atheist – he seems sympathetic towards Judaism and is said to have stayed in India for a time. Perhaps he likes Rabbi J, who was the Son of God only in a figurative sense, preached opposition to Rome, and claimed to be Messiah of the Jews only (or a Jewish sub-group only). The book weaves together two strands of alt-thinking popular back in 1971: Jesus as revolutionary, and Dead Sea Scrolls as final revealers of The Truth About Christianity.
Of course, the author isn't completely wrong. Scholars have also pointed out parallels between the NT and the Dead Sea Scrolls, although I think the idea today is that both reflect a common religious milieu, or turmoil, rather than Christianity literally emerging from within the Essene community. One similarity was that both groups used the Jewish apocryphon known as the Book of Enoch. It's also common to argue that Jesus was more “Jewish” than the later Church. Still, like all theories postulating that Paul changed the original message of Jesus, Lehmann can't explain why Paul would join the Christian movement in the first place if he didn't really believe it. The idea isn't impossible, but surely other options than “Paul stole the Jesus Movement” are also on the table?
That being said, “Rabbi J” is nevertheless a relatively good introduction to the Essene-Zealot speculations concerning Jesus, and I therefore give it four stars.