Probably my favorite book by Terry Pratchett. Whether you're a fan of his other books or not, a fan of Discwold or not, you have to love this book. It sort of makes me wish he did more stand alone books and books outside of the Discwold universe, actually. And I'm a huge fan of Discwold.
I have never read one of his books and not enjoyed it, and I've read a lot of them, but this is still my favorite.