Going Postal is a Diskworld novel, whatever that means. There is no mention of this round planet anywhere in the body of the book, and no clues in there whatsoever to hint at what makes this one of that particular series. Please do not enlighten me, I don't care. If I wanted to know I would look it up.
The spoilers start with the first two pages, so I won't even try to describe any details. In a nutshell it is a romp which takes a con man and a very unusual set of circumstances to their logical, or maybe that should be illogical, conclusions. As with most fantasy novels, this one plays fast and loose with the well-known rules of how and why fantastical creatures operate, makes up much of the rules as it goes, and tailors the magic to suit the author's needs and whims. Unlike most fantasy novels, it is compellingly well-written, and pieces of it are firmly rooted in science fiction (what if the post office's biggest competitor in a world with no electric power was a series of semaphore towers? What if those towers were not well maintained?). Like too many fantasy novels, the characters are caricatures but that's part of the charm of this book.
This book is self-contained, but I can see where it almost demands a sequel, and could have a prequel. Many prequels.
I liked it, because just often enough to keep me hooked, Pratchett throws in an oblique jab at or homage to another author, or a movie, or some other piece of fannish trivia. They are piquant enough to make me LOL. A couple even made me squeal out loud, scaring other restaurant patrons. In its own twisted way the story is inspiring without moralizing. It features a Unlikely Hero and a benevolent tyrant. There is something approaching a love story. But mostly, it's just plain fun. Glad I read it.