I think part of the appeal of the book is that, despite its literary flaws, it suggests a larger mystery in the world that remains unsolved, and the clues to this "mystery" are publically-accessable, in museums, books, churches, etc. Plus, it is always titillating to read about secret societies, and if they are secret organizations within a respectable institution such as the Vatican, then so much the better. It's like the ultimate episode of the tv show "City Confidential." Throw in an orgy, er, uh... a holy sexual rite, and you got yourself a pot boiler.