The Third Policeman, by Flann O'Brien (one of the pseudonyms of Brian O'Nolan, 1911-1966) was written in the late 1930's, rejected by several publishers, and not published until 1967, a year after the author's death. It is a peculiar book. Some people describe it as comic, even wildly funny. I did not find that, at least not consistently, but I do concede that the writing is marvelously inventive and engaging; often amusing, if not laugh-out-loud funny.
The central character, who is also the narrator, having participated in a brutal murder, sets out to search for the black box that is supposed to contain the money of the murdered man. In addition, as a sort of sub-plot, he is an enthusiast for an obscure (fictitious) pedant named de Selby. He refers frequently to de Selby's hare-brained theories, and there are lengthy footnotes detailing the absurd arguments among commentators on de Selby's work. De Selby's ideas are, however, no more preposterous than the "real" events of the story. The book ends by turning back on itself, with its tail in its mouth, as it were, which puzzled me. The point of that reversal is explained in an excerpt from one of O'Brien's letters, quoted in the introduction to the edition I read (Dalkey Archive Edition, 1999), but I would not have figured it out on my own. (I usually read literary analysis only after reading the book; in this case, reading the introduction first would certainly have altered my perception of the book.)
I did find the book interesting and sometimes amusing, but I was not sufficiently enthusiastic to recommend it without qualification. Perhaps a good overall description comes from the same letter of O'Brien, where he says that writing of a world "where none of the rules and laws (not even the Law of Gravity) holds good, there is any amount of scope for back-chat and funny cracks." As a sample of that, here is the introductory quotation, ostensibly by de Selby, which may give a taste of what to expect. If it appeals to you, you may well love this book: "Human existence being an hallucination containing in itself the secondary hallucinations of day and night (the latter an insanitary condition of the atmosphere due to accretions of black air) it ill becomes any man of sense to be concerned at the illusory approach of the supreme hallucination known as death."