I Shall Wear Midnight is the 38th Discworld novel and the fourth following the story of Tiffany Aching, young witch of the Chalk.
Reading modern Pratchett, I'm sadly, acutely (and morbidly) aware that, as he succumbs to Alzheimer's disease, these are the likely to be some of the last Discworld novels written. With the last few, this observation has been entirely external to the novel. With Midnight, however, I got the impression that the awful disease has finally started to affect the prose itself.
I could just be jumping at shadows, but I got the impression that he repeated himself a tad more often than usual. With such a long series of books, an author has the difficult job of balancing new and established readers in the same text: as such, as an established reader, you get used to having some back-story with which you must be patient. This necessary background could have contributed to the impression of repetition, but I'm fairly sure doesn't explain all of it.
An enjoyable story, and possibly the one to bring the Aching arc to a conclusion. She stands on her own two feet, proving herself a a capable Witch. There weren't any earth-shattering new concepts or new characters or unexplored areas of the Disc for fans to map out with their minds; non-the-less it was a satisfying read.