The Christ Myth Theory and Its Problems is probably the most credible and complete defense of the thesis that Jesus never existed to be written this century. The author, Robert M. Price, is an accredited scholar and fellow of the Jesus Seminar, and is widely read and advanced in the debate over Jesus' historicity. Here are my thoughts on his book.
First, it might be more accurate to call the book "The Christ Myth Theories and Their Problems." I know, it's not quite as catchy of a title, but I make this comment only to bring it to light that there are many theories that entail that Jesus never existed (as Price discusses within the book) and that each come with their own set of problems. For example, Price discusses an older theory that Jesus was a sort of development on a hypothetical Joshua cult from within Judaism, noting that the primary problem with this theory is that there is no evidence at all that there ever was a Joshua cult.
The book is primarily devoted to showing that almost all of the material in the gospels has deep parallels with Old Testament scripture, which is significant because, in Price's view, the early Christians were "'discovering' for the first time what Jesus the Son of God had done and said 'according to the scriptures' by decoding the ancient texts [Old Testament]." In other words, the entire life of Jesus portrayed in the gospels is no historical memory or oral tradition but only the results of unusual Old Testament interpretation. Price's analysis is exhaustive and far more extensive than I have seen in any other book on the Christ Myth theory (something that is lacking greatly in books like Earl Doherty's The Jesus Puzzle).
Price shows remarkable fair-mindedness in assessing the evidence for Jesus' existence, as he dedicates an entire chapter to discussing Paul's reference to James as "the brother of the Lord" going so far as to call it "The most powerful argument against the Christ-Myth theory." He surmises 3 different theories to account for James so-called kinship with Jesus that would not entail a physical relationship with Jesus, which are the following:
"First is the possibility that James was understood, like Thomas, to be the earthly, physical counterpart to a heavenly Jesus. Second is that James was prominent among the missionaries known as 'brothers of the Lord.' Third is that his fraternal connection is fictive and presupposes the historicization of a heavenly Jesus and seeks retrospectively to co-opt the James sect by subordinating its figurehead to Jesus as his brother."
These are not ad-hoc hypotheses, as Price adduces some circumstantial evidence for each of them.
Overall, The Christ Myth Theory is a good read and a must have for anyone interested in the idea that Jesus never existed. You can purchase it on Price's homepage.