Flann O'Brien, of course, was the pen name for Brian O'Nolan, a mid-20th century Irish author who was largely ignored during his lifetime, and who is sometimes considered a sort of Irish proto-Postmodernist.
In his 2009 review of O'Brien's Complete Novels, Fintan O'Toole gave this appraisal of O'Brien's convoluted first novel, At Swim-Two-Birds:
This is a book that begins by questioning why a book should have just one opening, and proceeds to give us three. It is a book by a man (Brian O’Nolan) who invents an author (Flann O’Brien) who is writing a book about an unnamed student narrator who is writing a book about a man (Dermot Trellis) who is writing a book. The narrator openly declares that “a satisfactory novel should be a self-evident sham” and that “the modern novel should be largely a work of reference,” since virtually all characters have already been invented. Its governing caprice is that fictional characters do in fact already exist, have independent lives, and are capable of revolting against the author who seeks to deploy them. The novel is a treasure house of brilliant pastiches of everything from Gaelic sagas and Irish folkloric narratives to the Bible, Victorian encyclopedias, scholasticism, pub poets, cowboy novels, and trashy thrillers.
You can listen to O'Toole talk about O'Brien's life and work in this New York Review podcast.
The most beloved book by O'Brien is one that was never published during his lifetime: The Third Policeman. It's a truly wondrous and bizarre novel over the course of which it becomes clear that the nameless narrator is in some sort of circular chain of events--is, in fact, dead and trapped in some sort of parallel hell, where the same disturbing inexplicable things occur over and over again.
In this Hirshhorn podcast, you can hear David Wilson, founder of the Museum of Jurassic Technology, read a passage from The Third Policeman featuring an infinite succession of nested wooden boxes. Worth your time.