"Foucalt's Pendulum" (Umberto Eco)
He has a lot of fun with this idea. And sends them all up: Templars, Rosicrucians, the Priory of Sion, the Masons, the whole lot of 'em. The Da Vinci Code guy definitely read this.
This book is highly recommended for Zoners, Eco brings up one conspiracy after another, documents it magnificently with a tour de force of scholarship, then demolishes it completely with the next one. By the time he's done with you, you realize anyone can prove anything they want if they know their way around a library.
Let it be a lesson to us all, you can sometimes prove or disprove the existence of a thing or an event with research, but not the validity of an idea. That's because ideas have no real existence except in our minds.