I will keep repeating the same fiction book whenever this question comes up until all 19k of you have read it:

The Dispossessed by Ursula K LeGuin

Iraq +100 is a collection of short stories by Iraqi authors set 100 years after the 2003 invasion of their country.

Perdido Street Station by China Mieville , The Quantum Thief by Hannu Rajaniemi and This is How You Lose The Time War by Amal El-Mohtar and Max Gladstone are all beautiful, gripping books that mess with narrative structure and conventions and deal with leftist ideas in interesting ways.

What was it you liked about The Third Policemen that you're looking for more of?

I really enjoyed the "Bobiverse" books if you like stuff about AI and space

I just finished reading A Confederacy of Dunces and really enjoyed it. I found its humor pretty funny, for a book written in the 60s, and the intentionally buffoonish protagonist is like an archetype for the "return to tradition" type guys, if taken to its most ridiculous extreme.

Also, it was published posthumously because the author committed suicide at 32, in part because of his spiraling mental health after the novel was rejected by publishers when he originally submitted it. Really tragic case of a creative not getting their due in life, especially considering the main character of the novel is in-part an exaggerated portrayal of what he saw as his own flaws.

Hey, that may have been me who recommended it. Awesome.

I wish I could say I had a dozen more books in that vein but there's really nothing that is perfectly comparable. Something else that comes to mind as a weird, propulsive narrative with a logic all its own, however, is Tom McCarthy's Remainder. Maybe Jean-Pierre Toussaint's early work would suit you, too - Monsieur, The Bathroom, etc. That's just me trying to remember other unconventional novels I read around the same time I picked up The Third Policeman, though, I don't know that they'd hold up as "similar" under more detailed scrutiny.

De Selby shows up as a character in another work of Flann O'Brien's, The Dalkey Archive, but that one didn't do it for me the way Third Policeman did. Still clever as hell, though. At Swim-Two-Birds is a bit too metafictional, but plenty of people who aren't me absolutely swear by it.

Dalkey Archive Press is named after the O'Brien novel, and they publish all sorts of amazing, weird, experimental fiction. David Markson? Alasdair Gray? Jon Fosse? Michal Ajvaz?

What are some other novels you've especially enjoyed?

I also second @marxisthayaca 's Bolaño recommendation.

(Edit - I see that there are several people on this site who have recommended The Third Policeman, which pleases me greatly.)

Another edit, to add The Invention of Morel, by Adolfo Bioy Casares.

The Third Policemen

Maybe Savage Detectives would be a better rec

Ancillary Justice and it's sequels are very good books about imperialism and angsty spaceships.