If you were fascinated by Illuminatus!'s mind-bending, interconnected conspiracies and overlapping storylines featuring characters like Hagbard Celine and Joe Malik, you'll love Gravity's Rainbow. Pynchon's novel is a labyrinthine journey through WWII Europe, where secret organizations, cryptic symbols, and paranoia abound. The plot spirals through multiple perspectives and timelines, challenging you to piece together its grand puzzle.

If you enjoyed the dizzying layers of conspiracy and secret societies in Illuminatus!—from the Discordians to the Bavarian Illuminati—Foucault's Pendulum offers a sophisticated, intellectual take. Eco's protagonists invent their own conspiracy as a game, only to find it spiraling out of control, blurring the line between reality and fiction with sly wit and historical depth.

If the anarchic, surreal humor and satirical jabs at authority in Illuminatus! made you laugh, Catch-22 will deliver more of that dark, irreverent wit. Heller's cast of eccentric soldiers—especially Yossarian—navigate a bureaucratic nightmare with a blend of absurdity and cynicism that mirrors the comedic chaos of Shea and Wilson's world.

If you loved the way Illuminatus! fractures reality with shifting timelines, footnotes, and multiple perspectives, House of Leaves will intrigue you with its labyrinthine structure and unreliable narrators. The novel's story-within-a-story format and typographical playfulness create an immersive puzzle that unravels as you read.

If the philosophical undercurrents in Illuminatus!—questioning reality, perception, and the nature of truth—hooked you, VALIS is a must-read. Dick's protagonist, Horselover Fat, embarks on a quest for cosmic meaning after a mystical experience, blurring sanity and insanity in a search for deeper truths, much like the existential questions threaded through Shea and Wilson's work.