Adam Weishaupt's Bavarian Illuminati lasted less than a decade in the historical record, founded in Ingolstadt in 1776 and banned by government edict in 1785. It has lasted far longer in fiction, and The Illuminatus! Trilogy is the book most responsible. Robert Shea and Robert Anton Wilson wrote it between 1969 and 1971, and Dell published it in 1975, three doorstop novels of drugs, sex, science fiction and every conspiracy theory the authors could find, stitched around one outrageous premise: what if every conspiracy theory ever proposed were simultaneously true.
The trick that makes the book more than a joke is where it plants its feet. Ingolstadt, the real Illuminati's real place of founding, is a real setting in the novel, not an invented stand-in city. Weishaupt is named. The order's actual dissolution is part of the backstory the authors build from rather than discard. Everything after that is openly, gleefully invented, up to a running joke that Weishaupt faked his death and assumed George Washington's identity. The novel does not pretend its wilder claims are history. It uses real history as the diving board.
That distinction matters for a society whose real history is often crowded out by exactly this kind of fiction. Most people who have heard of "the Illuminati" today know the conspiracy-theory version first and the eighteenth-century Bavarian one, if at all, second. The Illuminatus! Trilogy did not start that confusion, but it is one of the most influential single texts responsible for cementing it into pop culture, precisely because it used the real name, the real place and the real dates rather than inventing something unconnected to wear the label.