It is based upon a theory about Mary Magdalene being the bride of Christ, and that she bore offspring.. blood descendants of Christ who are alive today. It purports that the Roman Catholic Church maligned Mary because they viewed living descendants of Christ to be a threat to their authority, that she was not a harlot as they implied, but was a noblewoman of the House of Benjamin. It professes that the Templars were guardians of the secret of the San Griel (Holy Grail). It professes that San Griel was a catch phrase whose real meaning was Sang Riel or "Blood Royale", or Royal Blood... the living vessel containing the blood of the Savior.
The RCC considers such things to be heresy. I can see why, but am unsure why those who are not catholic would be so offended by the theory. I do not buy his story about how things went down.. but he DOES touch on some interesting history.
I haven't read it either, but as I understand, it presents an alternative ending to the gospel story whereby Jesus gets married and the church is complicit in hiding this from the world.