Brown's book is based on a book "Holy Blood, Holy Grail" by Michael Baigent, which I have read. It purports to show that there is an ancient conspiracy going on to this very day fostered by the Priory of Sion, an offshoot of the Knights Templar of medieval crusades fame. An interesting book, but full of supposition and speculation with no real documentary back-up. All of Brown's ideas come from this book.
There is a sequel by Baigent called "The Messianic Legacy". I haven't read it yet.
I just got two more books yesterday on this subject. One is "Breaking the Da Vinci Code: Answers to the Questions Everbody's Asking" by Darrell L. Bock. He has a PhD. in theology. His book is supposed to refute Brown's theories.
The other is "Secrets of the Code: The Unauthorized Guide to the Mysteries behind the Da Vinci Code" by Daniel Burstein, Editor. The book is an anthology of articles and essays on the "Code" by many theological luminaries, including Baigent, Karen King, Elaine Pagels, et al. By the looks of it, some are pro, and some are con. I'll know more after I read it.
Of course, Brown's theories are opposed by mainstream churches, especially the Catholic Church, and it's Opus Dei organization, both of with are not portrayed kindly, both it his "Code" book, and his earlier bood, "Angels and Demons".
So you work in a bookstore. Go check them out.