Overview:

Foucault's Pendulum (original title: Il pendolo di Foucault) is a novel by Italian writer and philosopher Umberto Eco. It was first published in 1988; the translation into English by William Weaver appeared a year later.
Foucault's Pendulum is divided into ten segments represented by the ten Sefiroth. The novel is full of esoteric references to the Kabbalah. The title of the book refers to an actual pendulum designed by the French physicist Léon Foucault to demonstrate the rotation of the earth, which has symbolic significance within the novel.
Bored with their work, and after reading too many manuscripts about occult conspiracy theories, three vanity publisher employees (Belbo, Diotallevi and Casaubon) invent their own conspiracy for fun. They call this satirical intellectual game "The Plan," a hoax that connects the medieval Knights Templar with other occult groups from ancient to modern times. This produces a map indicating the geographical point from which all the powers of the earth can be controlled—a point located in Paris, France, at Foucault’s Pendulum. But in a fateful turn the joke becomes all too real.
The three become increasingly obsessed with The Plan, and sometimes forget that it's just a game. Worse still, other conspiracy theorists learn about The Plan, and take it seriously. Belbo finds himself the target of a real secret society that believes he possesses the key to the lost treasure of the Knights Templar.
Orchestrating these and other diverse characters into his multilayered semiotic adventure, Eco has created a superb cerebral entertainment.
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Side Note: This book provided one of my favorite quotes:
“There are four kinds of people in this world: cretins, fools, morons, and lunatics…Cretins don’t even talk; they sort of slobber and stumble…Fools are in great demand, especially on social occasions. They embarrass everyone but provide material for conversation…Fools don’t claim that cats bark, but they talk about cats when everyone else is talking about dogs. They offend all the rules of conversation, and when they really offend, they’re magnificent…Morons never do the wrong thing. They get their reasoning wrong. Like the fellow who says that all dogs are pets and all dogs bark, and cats are pets, too, therefore cats bark…Morons will occasionally say something that’s right, but they say it for the wrong reason…A lunatic is easily recognized. He is a moron who doesn’t know the ropes. The moron proves his thesis; he has logic, however twisted it may be. The lunatic on the other hand, doesn’t concern himself at all with logic; he works by short circuits. For him, everything proves everything else. The lunatic is all idée fixe, and whatever he comes across confirms his lunacy. You can tell him by the liberties he takes with common sense, by his flashes of inspiration, and by the fact that sooner or later he brings up the Templars…There are lunatics who don’t bring up the Templars, but those who do are the most insidious. At first they seem normal, then all of a sudden…”