The Holy Blood and the Holy Grail

This is the book that started all the controversy and what has evolved today as The Da Vinci Code.
In summary, the authors argue that there is evidence that Jesus married Mary Magdalene, had one or more children, and that those children or their descendents emigrated to what is now southern France. Once there, they established what became the Merovingian dynasty, which is championed today by a secret society called the Priory of Sion.
[Extracts from http://en.wikipedia.org/wiki/Holy_Blood%2C_Holy_Grail]