Although this book contains some interesting historical material I wasn't familiar with (which redeems it somewhat), overall it has to be described as utter tosh.
The authors extrapolate from the wildest speculations to tell a story of their own fabrication, based largely on absurd parapsychological claptrap, which they regard as 'evidence'.
If you're looking for 'The Truth', as stated in the book's subtitle, you won't find it here.
This is particularly disappointing, of course, as the original story is still intriguing, and deserves some serious and intelligent research. Hopefully any renewed interest generated by this book will attract someone who can do the job properly.