Just in the process of reading it.
The basic argument:Christianity as we know it was shaped ~3rd century C.E. and at that time it was decided that certain common myths and archetypal elements of the hero's journey be personalised and embodied in the life of Jesus. So, elements including divine ancestry, a virgin birth, a guiding star as well as the sermon on the mount, the torture and crucifixion were taken out of the general mythical context and applied to the life of Jesus. According to Harpur, we actually have very little evidence regarding what Jesus's life might actually have been, but it seems likely it has little to do with the story that has been passed down, because all of the above elements (and more) pre-existed the Christian story and can be found, for instance, in Egyptian mythology.
From my understanding, Harpur feels that when these elements exist in a common "mythosphere" [my coinage], we can apply them to ourselves and derive power from them. By literalizing them and embodying them in an individual's life, early Christianity created a paradigm against which everyone would fall short--and robbed the myth of its metaphorical power and resonance.
My concern with the book: Harpur does state at the outset that this is meant to be a widely accessible work rather than a scholarly piece of writing. Still, given what little I do know about the early texts, a lot of the information that is derived from them seems sketchy and open to a wide margin of interpretation. Some of his neat little etymological progressions [Osiris=el Asar (Hebraic version)=El Asar-us (Latin suffix)=Lazarus] seem a little suspicious and overly convenient. I cannot help but wonder whether some of his other connections are also that way. Also, his version of the Horus myth is very different from the one I'm familiar with. There are often different version of myths out there, but still, it makes me wonder how much it has been tweaked to fit in with certain arguments.