Robert Anton Wilson's "Illuminatus! Trilogy". I read it as an arrogantly close-minded teenager, and it shocked my brain open, planting seeds of heretical ideas and omni-directional agnosticism that blossomed over the course of years.
I was an arrogantly agnostic and intellectual young adult when I read it. It had similar results.
I think the appendices may have been most important to me, ultimately. Some more than others of course.
Every now and then I return to it and the painfully sixties parts get even more painfully so - I suppose I shouldn’t expect an subtle treatment of women from a couple of guys who were working at Playboy when putting it together, but it’s still grating, and makes it harder and harder to recommend to the next generation.
Also it inoculated me against Ayn Rand with its parody of her extended paeans to the joys of being fucked in every possible way by rich assholes who value money over morals, and after seeing the damage done to society and people by libertarianism, I cannot thank Wilson and Shea enough for that.
https://en.wikipedia.org/wiki/The_Illuminatus!_Trilogy