I have read all of DBs books. Da Vinci is loosely based on Holy Blood Holy Grail 1985 by Baigent Lincoln Leigh (Amazon). The guy murdered in the Louvre in DVC is Sauniere, named after Berengiere Sauniere, who came into the "Cathar" treasure and renovated Rennes le Chateau in Languedoc. I visited there in 2012 and was creeped out by the Baphomet in the doorway. However upon visiting this summer, I noted that Baff himself had been vandalized - beheaded! RlC is a creepy place indeed.