Unfortunately, Baigent takes the reader on a bit of a "wild goose chase" in this book ... it does have a "payoff" that fits the title and general theme, but in a frustrating at-3rd-hand manner. Frankly, the book follows a pretty solid "story arc" for the first half, starting with stories of how difficult it often is to get access to obscure documents, and how they have (even after having been seen and photographed) a bad habit of disappearing behind "stonewalling" sources. The narrative then moves to the predictable South of France with the Cathars and the much-written-of Rennes le Château, discussing the brutal suppression of the former and the mysterious wealth and influence of the latter. There is (as detailed in his earlier books) a tradition that Mary Magdalene (Mrs. Jesus) at least had re-located there sometime in the mid first century, with the implication being that "hidden knowledge" persisted there about the "non-canonical" truth of Jesus, knowledge that the Catholic Church especially would go to extreme lengths to eliminate.
At this point the focus shifts to Israel, and looks at the context of the Biblical narrative. Baigent puts an interesting "spin" on things here that I don't recall previously seeing ... that Jesus, while being promoted by the Zealots as having royal/priestly bloodlines, was not particularly "in their camp", using for evidence his allowing women into his inner circle, and the whole "render unto Caesar" tax thing. There is a suggestion that the Judean Jesus was "willing to play ball" with Rome, much as the "historian" Josephus managed to do later (in switching sides from being a Zealot commander to a Roman ally). Baigent then looks around at where Jesus might have been over the "missing" 20 or so years of his life and settles on Egypt, where there had been a thriving Jewish community (and various schools) at the time. Much of the teachings of Jesus could come from assorted Egyptian cults, and was certainly not in lock-step with the Temple-centric orientation of his brother James and other close associates.
It's here, though, that things get "spotty". Baigent jumps from place to place and tradition to tradition to illustrate possible sources/influences on the teachings of Jesus, but without much real linkage. He even spends a very interesting chapter dealing with a subterranean complex in Baiae, near Naples in Italy. While this does sound very much like an ancient initiatory temple, and possibly the origin of the "River Styx" mythologies, it has nothing really to do with the Jesus story, except to nudgingly suggest that it might because of there having been an ancient Jewish community nearby.
Perhaps it's my personal familiarity with this sort of thing, but it seems odd to me that Baigent would spend a good third of the book just trying to establish that there were initiatory cults in the ancient Mediterranean! It seems a long way to go to be able to suggest that many of the sayings attributed to Jesus are in close relation to what has been preserved of these cults. Yes, the references that could be made to Lazarus and Mary are fascinating, but are ultimately only suggestive.
Of course, the Vatican is the big baddie in this, being the spawn of the "Greek" tradition of Paul rather than the "Jewish" tradition of James, etc. ... for the Catholic orthodoxy to have any leg to stand on, Jesus has to be a unique God-man whose brief tenure on Earth is the singular defining point in human history (let alone moronic modern American "evangelical" movements which need Jeeeezus to be GOD, complicating doctrines like trinitarianism be damned). To have solid proof that Jesus survived the crucifixion, and went on to "cause trouble" in other parts of the Roman Empire would be a Very Bad Thing for The Church of Ratzinger (and the book is recent enough to note his elevation from being the head of The Inquisition to the current Pope), the clear reason that Rome has constantly endeavored to keep things like the Dead Sea Scrolls, the Nag Hammadi library, and similar writings from the public consciousness. The Islamicists also feature in the "villain" column, as the insane attempts to assert that the Jews were not in Palestine for millennia have frequently led to summary destruction of any ancient buildings or document caches which offer proof of a thriving, wide-spread Jewish presence all over the middle east.
This point leads Baigent back into the "text trade", the shadowy underground of pilfered ancient scrolls and fragments, in which he resumes his search for "The Jesus Papers". While he is unable to back-track through his initial sources (interestingly, Catholic historians and theologians) who claim to have encountered documentation that "proves Jesus was alive in 45ce", he does, after a twisting, shrouded, and vague-on-details search get to hold in his hands a letter from 1st Century Sanhedrin court records of a bani meshisha ("Messiah of the Children of Israel") formally responding to accusations that he claimed to be the "Son of God", in which it's explained that he means, filled with the spirit of God, and not in anyway divine. Baigent gets to see (but not photograph) this in stabilized-atmosphere glass mountings in a room-sized safe in a European city in the presence of their owner. Needless to say, that sort of thing could change history.
Anyway, with the caveats expressed above, I highly recommend Baigent's The Jesus Papers to anybody with an interest in looking behind the veil of lies which is Christianity ... it's not the book that I would have hoped for, but it has a whole lot of material to recommend it! The Amazon new/used guys have "like new" copies of this hardcover edition for under a buck (plus shipping), and there are "good" copies of the paperback going for as little as 1¢ ... I'm glad to have it in my library, and there were a good half dozen books referenced in it that I'm going to have to check out!