So the (anachronistic) elevator pitch for The Illuminatus! Trilogy is "Foucault's Pendulum on acid." It's a kaleidoscopic, kitchen-sink, semi-sci-fi conspiracy novel whose 30,000-year-old intrigues encompass (but are certainly not limited to): the lost continent of Atlantis, the 1968 Democratic convention, Atlas Shrugged, H.P. Lovecraft's Elder Gods, talking dolphins, John Dillinger, the number 23, the goddess Eris, and an undead army of Nazis. And the Illuminati, of course--though the history and mission of the organization goes through many different permutations and layers of deception. I'm pretty sure I still don't get it, but then, I don't think I'm supposed to. It's not really that kind of book, much more a ride than a destination.
Its flaws I attribute almost entirely to the era in which it was written--1969-71, though it wasn't published in the omnibus form I read until 1984. It's awash in hallucinogen-as-spiritual-practice nonsense, and its female characters are almost entirely sexually generous to an eye-rolling degree. (I.e., there are a LOT of impromptu blowjobs in this book.) There's also the authors' uncomfortable tendency to refer to black characters' race nearly every time they're mentioned. Whether these faults are minor or major I leave up to the individual reader. But it's largely great fun, and its cult status is well earned. The couple I borrowed it from has a copy each.