As some other people have mentioned, various editions of this book have introductions or afterwords that destroy much of the pleasure of reading it; I definitely advise reading just the text of the book, skipping anything before or after, even if it's the author's own words excerpted from a letter or something.

This is a wildly imaginative, funny, grotesque, confusing, mind-bending book in which things happen from their own crazy internal logic, mundane notions of making-sense be damned. For me the enjoyment here is about experiencing the craziness, its shapes and textures, maybe thinking about the various things it might mean, what might be going on, what other crazy things it's similar to or different from, and generally experiencing the waves and whorls of uncertainty that it produces. (So the warned-against front and back matter, which say very blandly "what is happening is X", interfere significantly with that!)

Images from this book will stick with me forever; the ever-smaller set of boxes, the elevator that weighs you on the way down, the careful placement of bicycle parts, the works of de Selby, the soul named Joe... Well! You should read it for yourself; highly recommended.