Looking at the list of Discworld novels inside the cover of this particular volume, I saw that it, like all the Tiffany Aching books, was noted as being 'for young readers'. All through reading the book, I wondered what it was that made it 'for young readers'. Were the concepts less complex? The language less advanced? Mature subjects carefully excised? Eventually I decided that the only thing that made "I Shall Wear Midnight" 'for young readers' was the fact that the narrator was not quite sixteen.
Classification by age of narrator is something of a bugbear for me, since my novels sometimes have both a teen and an adult narrator and thus apparently don't quite fit. Ah well.
Thoroughly enjoyed the book. There's very few long-running series which get better as they go along, but Discworld is definitely one.