"All descriptions of artwork, architecture, documents, and secret rituals in this novel are accurate." - from
The Da Vinci Code novel.
Also, from the the article I cited in my earlier post:
"Although [the author] declines interviews now, he told National Public Radio during a 2003 publicity tour that the book's characters and action are fictional but that 'the ancient history, the secret documents, the rituals, all of this is factual.' He also told CNN at that time that 'the background is all true.'"
However, according to this article
http://www.aboutbibleprophecy.com/davinci.htm
"As for Brown's claim about the Dead Sea Scrolls - these scrolls were found in 1947, not in the 1950s as Brown mistakenly claims on page 234 of
The Da Vinci Code."
This and other sources point out a number of historical errors in the novel.
Whether deliberate or unintentional, the mixing of real historical figures and events with fictitious "facts" is a shrewd marketing move. The genuine history lends the book an air of authenticity ("Hey, the Emperor Constantine and the Council of Nicea were REAL!"), but the author can always take cover behind the "fiction" label ("So what if the 'Gospel of Philip' isn't really in the Dead Sea Scrolls! It's FICTION!").
Unfortunately Dan Brown didn't explicitly distinguish fact from fiction from speculation (probably because it would hurt sales), so confused readers have to get the truth elsewhere.