That's not a defense of Dan Brown, it just means he has poor taste in kooks. There is crazy awesome and there is crazy boring, and even Garth Ennis couldn't make the grail stuff fun when he used it in Preacher. Now, give me novel about Dion Fortune, Ian Flemming and Aleister Crowley summoning King Arthur on the white cliffs of Dover to stop Operation Sea Lion and I'll forgive all sorts of awkward prose.
How 'bout a book about the founder of JPL using Crowleyan magick to summon a storm to destroy L. Ron Hubbard and his ex-girlfriend? Or a book about the guy who wrote James and the Giant Peach spying on the US for Britain and influencing the American war effort by flirting with Eleanor Roosevelt? Best I can do. They both happen to be true, so I doubt there's a decent work that puts them together, but you never know. I won't recommend Da Vinci Code to anyone. It's basically a Rocky & Bullwinkle novel. But as Rocky & Bullwinkle novels go, it has the most Jesus in it of any I know.