I Shall Wear Midnight is Terry Pratchett's fourth Tiffany Aching book. If this means nothing to you, then you are in the wrong place. Go read about The Wee Free Men, and then read the book, and then read the other books in the series, and THEN you can come back here.
This is certainly a good book, but be warned, it does not start out as a good book. It is surprisingly average and a bit uneven and unpredictable in its writing style for the first 50ish pages (and perhaps again at the end). In 2007 Terry Pratchett was diagnosed with a form of early onset Alzheimer's disease, and by 2010 he was dictating his books to be transcribed by his assistant. This may explain the odd changes in style, but thankfully once he gets into the groove things flow nicely.
The story starts off with Tiffany back on the Chalk, where she is finally serving her own steading. This is turning out to be harder than she might have expected, and she is running herself ragged (not that she would ever admit this). Things are generally taking a turn for the worse; Roland (her Baron) gets engaged, the old Baron dies, and after a violent death in the village nasty rumors start to go around -- after all Tiffany was with the old Baron when he died, and she was the first one there when the child died... It emerges that the rumors are a bit more than rumors, and are being created by an evil force that has it out for witches in general and Tiffany specifically.
The driving plot of the story is Tiffany's battle with the evil force, who we find out is known as The Cunning Man; he is an evil being that reappears periodically to try to destroy all witches. The story takes a long time to reach this point however, because Tiffany's struggle to be a proper witch in chaotic times, and in a place that isn't used to having a witch, is a major subplot. Perhaps even two or three separate sub-plots.
The book covers a lot of ground, as Tiffany visits Ankh-Morpork, meeting a number of familiar Discworld characters, and most of the well-known witches (Nanny Ogg, Granny Weatherwax, Eskarina Smith, etc.) pop into the story at least once. Of course, the Nac mac Feegles are present, and all the regular Tiffany story characters. The story has a strong feeling of being a tour-de-force showcase, and perhaps, a final ending to the Tiffany story. We can only hope that that is not the case.
I Shall Wear Midnight is the 38th Discworld Novel, and the fourth Tiffany Aching novel.
In the Discworld series it is preceded by Unseen Academicals and will be followed by Snuff.
In the Tiffany Aching series it is was preceded by Wintersmith, and there is currently no planned followup.