Tiffany Aching is a witch. This mostly means that she helps women give birth, cuts old ladies' toenails, bandages wounds and helps the dying in the area whose witch she is. She has first sight and second thoughts, meaning that she sees what is really there and thinks twice about what she sees. She does the things that need doing, but that few really think about or get round to doing. People respect her, but lately that respect seems to have changed to fear and suspicion. There are tales about old, eccentric ladies all over the countryside being hounded and killed by paranoid villagers.
The old baron dies, and Tiffany has to go to Anhk-Morpork to give Roland, her childhood friend, the bad news and fetch him back to Lancre. Tiffany once saved Roland from the Queen of the Fairies, but he doesn't remember anymore. Now he has an excessively sniffly fiancée and a battleaxe of a future mother in law, and seems to be making all the wrong decisions. He is suspicious about Tiffany's role in his father's death, and Tiffany finds herself locked up in the dungeon, right next to where they keep the goats.
Something old and very evil is spreading its influence, and Tiffany needs to find a way to stop it, before it gets more innocent old ladies, and herself, killed. She can't ask for help from her fellow witches, as once she's shown herself incapable of dealing with such things, who is really going to trust her to manage anything at all without assistance in the future?
I Shall Wear Midnight is the fourth, and according to Terry Pratchett, final Tiffany Aching book. It's the first of the books to really integrate more into the wider Discworld universe, Tiffany leaves Lancre for a while, goes to the capital, runs into familiar characters from the City Watch. It may be a book written for young adults, but it's also a dark book, and Pratchett does not shy away from writing about the harsh realities of life. The book covers things like domestic abuse, miscarriages, neglect, false accusations, wrongful imprisonment, jealousy and thoughtless hatred. Pratchett never underestimates his audience, and that is probably why he is such a great author, whether he's writing for children or adults.