Chromatose
Squid pro quo
After taking it to court, the judge ruled that no, he did not in fact rip off "Holy Blood, Holy Grail." While they cover indubitably similar topics, I'd say Derek's statement of it just being a general formula of "conspiracies" and "esoteric groups," as the specifics in question, have been tossed around for ages. As far as literature is concerned, theirs was equally fiction, but Dan Brown's "dumbing it down" and selling millions made them a tad jealous.