One of the many posthumous Baen-collections of Keith Laumer’s work, Odyssey collects two novels (one great and one average if fun) and a few shorts stories, most of whom are also rather average with one exception. A Trip to the City (aka It Could Be Anything) starts simple and then descends into a sort of madness that only the best pure-concept-based stories can do achieve, yet the story works even on the narrative level.
It’s empiricism driven to the extreme: anything you believe to know about the world, that you haven’t experienced yourself but rather taken in as part of your larger culture, turns out to be wrong. A boy/young adult leaves his hometown only to arrive in a city that is hollowed out by strange beings who have taken over most of the population.
If that weren’t enough, he meets another person who has never heard about our culture (Earth, the United States, etc.) and seems to stem from some fantasy world/middle age society, only that both of them are sort of wrong (and right) in the end, as the world really is not what any of them thought.
A Trip to the City is not especially complex or sophisticated, but on a pure mindfuck-level, it really succeeds admirably and makes you question how real anything beyond the reach of your everyday life really is.