An article from the Guardian details an ongoing lawsuit in the UK that pits the authors of a nonfiction book Holy Blood, Holy Grail against Dan Brown, best-selling author of the Da Vinci Code. The suit challenges Brown over his source material and claims that Holy Blood,Holy Grail is more than just a piece of "incidental" background, as was claimed by Brown's solicitor.
I haven't read Holy Blood, Holy Grail, so I can't speak to the totality of the situation, however the link at Amazon contains preview pages that sounded familiar as background and it does bring into question whether there is some merit to the claims.
My initial thoughts are that the work of non-fiction may provide well- researched events that were then used by Brown to build a framework for his characters to run around in. Certainly, the key plot elements of the chase and the murders are not in the work of non-fiction and are key to the enjoyment of Brown's book.
However, there do appear to be some elements, including some of the code handling, that are present in both.