Holy Blood, Holy Grail

Holy Blood, Holy Grail (originally The Holy Blood and the Holy Grail, retitled for the US) is a 1982 book by Michael Baigent, Richard Leigh and Henry Lincoln, which claims to expose the true secret of the Holy Grail.

The theory
The theory proposed in the book is:
 * Jesus Christ's death was faked.
 * Jesus and Mary Magdalene married, had children, and moved to a nice little gîte in the south of France.
 * Jesus' descendants became the Merovingian dynasty and, through them, the basis for all of Europe's royal families
 * This secret is that the "Holy Grail" is not, as traditionally accepted, a cup that once held the blood of Christ, but literally the blood of Christ itself. The idea comes from the similarity between the phrases, san greal (holy grail) and sang real (royal blood).
 * The Priory of Sion is a secret organization devoted to keeping this secret and helping Jesus' heirs.
 * The Roman Catholic Church is aware of all this and has been involved in covering it up.
 * The end goal is a pan-European government headed by one of these descendants; in particular, the current head of the house of Habsburg.

Dan Brown
Dan Brown's massive best-seller novel The Da Vinci Code stole borrowed many aspects of these theories. The theories are not generally accepted by historians, or even most readers of wild-eyed fiction. The biggest flaw with the novel (among countless many) is that it takes the forged Priory of Sion documents at face value. In fact, the Priory of Sion never existed except as a small secret society (read: bored circle of friends) created by one Pierre Plantard in 1956. Plantard and his friends then proceeded to forge numerous documents and plant them in the Bibliotheque Nationale de France.

Rat Scabies
The book Rat Scabies And The Holy Grail by music journalist Christopher Dawes is the probably-mostly-true story of Dawes and his neighbour Rat Scabies (Chris Millar), one-time drummer of punk rock band The Damned, going a-grail-hunting. It includes meeting Henry Lincoln, and recounts just how pissed off Lincoln was by Dan Brown monetising his life’s obsession and redigesting his tedious book (it really is terrible) into The Da Vinci Code. Rat likes the Dawes book ‘cos it means interviewers have to ask him questions about two things now. (It's also the only one of these three books worth reading.)