This is a sort of very basic overview of some of the major players in the most popular Conspiracy Theories, which I read in one sitting on the plane. It's really light in providing the interesting coincidental details that make Conspiracy Theories fun, but I suppose it makes an acceptable overview for someone who's never been exposed to them before. I did learn about a group I'd never heard of before. Kind of pointless unless you can find it in the bargain bin (like I did).
It covers the Assassins, the Bilderberg Society, the Bohemian Club, the Club of Rome, the Council on Foreign Relations, Essex Junto (very interesting -- didn't know about those), Freemasonry, the Hermetic Order of the Golden Dawn, The Bavarian Illuminati, the Knights Templar, the Ku Klux Klan (hardly secret, but whatever), the Mafia, Majestic-12/The Aviary/The Aquarium, Mensur, Opus Dei, the Order of Skull and Bones, the Priory of Sion, the Rosicrucians, the Round Table, Triads, and the Trilateral Commission.