This is the shortest of all the Discworld novels, and perhaps the least impressive. I have heard that it was originally a lavishly illustrated novel with art by Josh Kirby (not Jack Kirby of comics fame), but that edition was never made available in the US. Nevertheless, the story by itself was published, and therein lies the problem.
Because Pratchett is a master of writing about the human condition, and this novel does not focus on the human characters, but instead on some Hell-dwelling demons who are little more than ciphers. Worse, Pratchett breaks the flow of the novel by continually making overt references to Earth, which disrupts the flow of the novel. And though the novel is ostensibly a method of getting Rincewind out of the Dungeon Demons he was trapped in at the end of Sourcery, it is never truly explained how it was that he got from the Dungeon Dimensions into Hell, which are two distinct places.
I would like the opportunity to see the illustrated version, because if it's anything like The Last Hero (with Paul Kidby), the illustrations add to the jokes and potentially raise the overall humor of the book. As it is, however, this is the one Discworld novel I encourage you to skip.