Going Postal is a Discworld novel. I assume everyone is already familiar with PTerry's big zany series with its goofy but unexpectedly consistent cosmology. If you're not, though, this actually would be a good book to read on its own; some minor characters and of course the setting come from the earlier books, but I think it would stand on its own quite well. The new arrival in Discworld would miss a few references, but this story would certainly make sense.
Pratchett is definitely growing as a writer, in both style and substance. This is still a fast-paced, humorous book with footnotes, but in an unusual innovation for Discworld, it's actually broken into chapters. The chapters have chapter headings which are little bullet-point lists of the important things that will happen in the chapter, which would probably be a big help for people trying to quickly re-read the book. I don't know if they help the casual first time reader concentrate, but they at least didn't bother me. The fast paced story has a solid plot, with a climax that's somewhat unexpected, real character development, and meaningful exploration of some themes about evil and the nature of some parts of modern society, and along the way delightfully skewers obsessive collectors, secret societies, corporate executives, foolish religion, and of course the postal system. Longtime readers of the series will particularly enjoy exploring new depths of Lord Vetinari's character.
This is not the best comedy in the series, but it may well be the best story, and it's probably Pratchett's best written novel to date. I give it a 9 out of 10.