Harpur has an interesting idea, that the Christian religion is in fact a retelling of ancient Eqyptian myths, inventing an imaginary 1st century figure called Jesus as a character in stories that the myth-makers and their audience would realise were purely metaphors for spritual truths.
Harpur is able to suggest some parallels between Egyptian religion (to the extent that it is known) and Christianity, such as a concern with eternal life after death, a cross-like symbol (the ankh) and images of a mother with a child.
However, Harpur does not really introduce any evidence for these views, preferring to proceed by assertion rather than demostration.
His ideas are derived from earlier 'experts', who published in 1833, 1881-1907, and 1944-1997. Unfortunately, after presenting their ideas, all Harpur does is refer us to their texts for such evidence and argument that they might have presented. This makes it difficult to assess these claims.