This book sounds a lot like Doherty's The Jesus Puzzle. I don't think many scholars doubt that "pagan" mystery religions played a significant role in the development of the mystical Jesus of Gentile Christianity, such as what we find in Paul, John, and later Christian writers. Few scholars tho accept the additional stipulation that no historical Jesus lay at the core of Christianity. I personally find this argument very unconvincing. It pretty much assumes that Jesus was an invention of Gentile Christianity but this fails to account for the very existence of normative Jewish Christianity which had no conception of this "mystical Jesus" and which followed and prized the words of Jesus which the Pauline church paid very little attention to. James the Just's claim to authority as the leader of the Jerusalem Church as the blood brother of Jesus makes no sense at all if no historical Jesus existed -- and his conception of Jesus drew on none of the mysticism of Paul (in fact, the evidence suggests that James viewed Paul as a heretic). The Nazoreans and Ebionites, the descendants of the original Jewish followers of Jesus, were in fact condemned as heretics by the catholic Gentile church because they dared to hold onto their traditional teachings and failed to accept the Pauline and Johannine theories on the redemption and the divinity of Christ. The Islamic view of Jesus as a great prophet, but not God, in fact derives from the traditional Jewish Christianity of Judea and knows nothing of the developments of the Gentile Church. The claims of Doherty and Freke & Gandy, if I understand them correctly, would seem to be biased against the Jewish Christians who had little connection with the mysteries of Gentile Christianity and who viewed Jesus as a very human prophet whose words were passed through oral tradition and written gospels, and who were very unlikely to draw their traditions from Gentile Christians whom they viewed as the real heretics. All the evidence I've seen suggests that the direction of influence originally was from Jewish Christianity to the Gentiles, and not the other way around.
Leolaia