It took me a while to get around to this one because the Tiffany Aching books have never been my favorite Discworld series. But I was really surprised with this one. Maybe because Tiffany's older in this one (sixteen, I think, or almost sixteen), the book has a darker, more sophisticated tone.
Tiffany Aching is the witch of the Chalk, the only witch in the area. The people are still getting used to having a witch, and Tiffany's still coming to terms with the workload and the lack of help. When the old baron dies, Tiffany volunteers to go to Ankh-Morpork to notify his son, who's gone to the city with his fiancee and her mother. But people in the big city are strangely hostile toward witches, and Tiffany keeps seeing a monstrous vision of a man without eyes--a man who hates witches and seems to be following her.
The plot isn't particularly unusual if you've read a lot of Discworld books. It's well-done, though: entertaining, funny, touching, interesting, and funny (I put funny twice because no one's more consistently funny than Pratchett). The wee free men are of course part of the story, although without as central a part as in the earlier books. And while Pratchett often has his other major characters do cameos in other characters' books, there was a lot of that in this one, including one character I was very surprised to see (no, not Death. He's in all of them).
I think this is the last Tiffany Aching book, from what I've heard. I liked this one so much that I hope I'm wrong. I'd like to see what Pratchett does next with the character.