I actually read this one after "Witches Abroad", which comes chronologically later. But it never stopped me from enjoying either of the books. Which is in itself a sure sign of Pratchett's genius.
Plus: this is the one with the (IMHO) famous footnote on the theory of alternative universes, trousers and the significance of the invasion of Poland in 1939 in the greater scheme of things. I still remember it!