“Holy Blood Holy Grail” (HBHG) by Michael Baigent, Richard Leigh and Henry Lincoln, was published in 1982 and contains a lot of the interesting hypotheses that appear in “The Da Vinci Code” (TDVC), down to the breaking of Sangraal to either read “Holy Grail” or “Royal Blood”. I think I would not have been so gripped by TDVC had I read HBHG before.
This book is the result of 11 years of rather meticulous research by the authors into the mystery surrounding some documents discovered a hundred years earlier in southern France and the mysterious organisation that called itself Priory of Sion, ultimately leading them to question the very foundations of Christianity. This book caused a furore when it was released what with its explosive hypotheses like that of Jesus's mortality, that the crucifixion and the resurrection was staged, that Jesus was married to Mary Magdalene and had at least one child with his bloodline continuing to this day, etc. The authors present extensive cross references and analyses to support these hypotheses unlike some of the seedier books on such topics. If you have read The Bible with an open mind and without considering it to be the literal word of God but a set of interesting historical documents, this book would surely interest you. You should not be shying away from this book even if you are a rather pious and devoted Christian.
The book does get a bit tiring at times though and some of the meanderings seem a bit baffling and ultimately pointless (quite like in TDVC). In the end, I still do not know what was written in the documents unearthed in southern France or what the mysterious “Et in Arcadia Ego” is really supposed to signify. The Priory of Sion has apparently been established as a rather elaborate hoax, so the basic foundation on which this book builds its hypotheses has now been badly shaken. There are also quite a few dubious jumps of logical deduction (I do not see how their conclusion follows from the presented “facts”) and to make matters worse, these are taken to be the foundation for the next set of such deductions! Small wonder then that this book was dismissed by a large number of modern scholars. That did not stop it from becoming a bestseller and it would not stop me from recommending it - it never hurts to have an open mind and at least be aware of the alternative theories to the accepted dogma (which itself has rather dubious foundations).