SPOILERS for The Da Vinci Code
I’ve read only Brown’s Da Vinci Code, but he’s clearly drawing on lots of pre-existing sources. That doesn’t mean any of it’s true. The Knights Templar really existed, and Brown relates accurately the history/legend of the Templars – that they went to Jerusalem, when they came back they began a very successful recruiting campaign, that the order was extraordinarily wealthy and powerful, and that they were rounded up on a single day in 1307 and eventually the order was dissolved. But there are perfectly good mundane political reasons for their power and wealth other than the magic of the grail or hush money from the Pope to keep quiet about the Bloodline – the Templars were very popular because everybody loved crusaders, and that lead to an increase in numbers, which led to greater military power, which led to wealth from conquered lands (and enough political juice with the Holy See that they got to keep the use of their wealth). And similarly, there were perfectly good reasons for their fall; the secular royalty didn’t like such a large, rich, and heavily armed force spread throughout Christendom which didn’t answer to the Crown.
Similarly, “Holy Blood, Holy Grail” is a real book, and the San greal/Sang real connection has been around since the Middle Ages. Brown didn’t invent the Priory of Sion, either – it probably doesn’t really exist, most likely being a hoax developed in the 1950’s, but it’s been a regular part of Grail lore since then. Of course, the Freemasons are real, and at times may have been a force to be reckoned with (George Washington and a handful of other of the Founding Fathers were masons), but like most fraternal organizations today (the Elks, Knights of Columbus, college fraternities, Skull & Bones, etc.), it’s mostly a mixture of social club and charitable organization. Of course Opus Dei is a real organization of Catholics which has sometimes been criticised for its conservativism (and its embrace of self-administered injury as a pathway to righteousness). And finally, the Golden Mean is real, it shows up frequently in nature (although Conan link debunks some of Brown’s claims about it), and it’s important in art and mathmatics – I remember watching a filmstrip about it (starring Donald Duck, IIRC) in my math class in 1985.
Brown took all this stuff, jumbled it together, and came up with an enjoyable potboiler. It all existed before him, and people were writing conspiracy theories about it before him, and some of it (the masons, the Golden Mean) really exists, some of it (masonic power, sang real, the Priory) is almost certianly bull, but bull that’d been around for decades or centuries before Brown got his hands on it.
Personally I always loved this mysticism stuff, and I spent many hours in college when I should have been studying reading about the Klysty, Christian Rosenkreuz, and The Hermetic Order of the Golden Dawn, but, y’know, none of it’s true.
For anyone who’s interested in this stuff, I’d recommend Umberto Eco, specifically his book Focault’s Pendulum. Not an easy book the read because it’s clear that the author is so much smarter than you are, but it covers some of the same ground as Bown does, but with better research. Also the Gabriel Knight computer games from Sierra, if you can find them (the most recent was 1999), which, like Brown’s books, inform your investigation of modern murders with the history of secret societies and cults. (And the third game focuses on the Grail in particular.)