A general problem with the book is it is hard to separate argument, conclusion, and secondary extrapolation. Too much hinges on a too brief analysis of Paul's letters. There is little in the book that could stand on its own if Doherty's conclusions on these were rejected. Key statements about the Gospels - the Mark was not written until 85-90 A.D., that the first chapters of Luke were a later addition - are argued to briefly.
Doherty's argument concerning Paul is that Paul gives no indication of equating Jesus with a historical man. The problem is that he does, as in Galatian's where he mentions "James, the Lord's brother." Doherty proposes that this was meant figuratively, and I suppose it may have been, but he does not give convincing argument that it definitely was. Then, however, the argument becomes that if Paul had known Jesus was a historical person, he would have talked about him that way more often. Nobody, however, is saying that Paul viewed Jesus in purely historical terms - Paul claims to have seen him post-mortem and never met him in life, so it is not surprising that he refers to him as being historical only occasionally. The more surprising claim is that Paul had no notion of a historical Jesus, but just happened to say things that implied such a figure.
A good point of comparison might be C. S. Lewis' Mere Christianity. As I recall, mentions of the life of Jesus are sparse in the book, but Lewis certainly believed that Jesus was a historical figure. [I don't plan on re-reading the book for the sake of this one post, but if people can provide even partial lists of HJ references in the book, I would be thankful.]
Without a surer argument from Paul, the rest collapses. For example, Doherty mentions that there seems to be some connection between the Q and the Gospel of Mark. It takes a great deal of confidence in the verdict on Paul to deny that this connection could be a historical Jesus. The argument from the Q is also troublesome. That such a doccument existed may be the most probable explanation for correspondences between Matthew and Luke, but the fact remains that we do not have the doccument, so to make lots of definite statements about it is a dubious undertaking.
One part that comes close to standing alone is his argument that the whole passion account was created based on Hebrew scripture, but the argument is very weak. In his frantic search for the vaguest parallels between the Old Testament and the passion story, he ends up looking rather like Christian apologists who claim that similar passages predicted Jesus' story. Indeed, I suspect Doherty borrowed some of his citations from such apologetic works.
Though Doherty did not hammer parallels with other gods as much as I had expected, these do not prove very much. Consider the following passage from a book on the Native American Ghost Dance movement:
All the delegates agreed that there was a man near the base of the Sierras who said that he was the son of God, who had once been killed by the whites, and who bore on his body the scars of the crucifixion. He had now returned to punish the whites for their wickedness, especially for their injustice toward the Indians.That the figure described, Wovoka, has been mythologically mixed up with Jesus is obvious, but Wovoka was in fact a real person. It is entirely possible that Paul identified the real figure of Jesus with other gods of his day.
There are two points in Doherty's argument, "smoking guns," he calls, them, where he does not merely argue from silence, but they do not smoke as hot as he thinks. He claims Heb 8:4 says "If he had been on earth" but as Richard Carrier points out in his generally positive review of the book, the correct reading is "if he were on earth" (See Appendix 1, 10x). The other is a passage where a 2nd century Christian apologist appears to reject the idea of a historical Christ, but this is not nearly as clear as Paul's mention of Jesus' brother.
In sum, I must concluded that this book does not merit the attention it has gotten in atheist circles.