This is the third Discworld book, and the first with the witches. It introduces Granny Weatherwax, one of the main characters of the whole series.
At the start of the book a wizard passes on his staff to a baby that has just been born in a rural village. This child is supposed to be the eighth son of an eighth son, but it turns out to be a girl called Esk. The father and Granny Weatherwax hide the staff and try to ignore what has happened. As the girl grows and clearly has magic growing within her, Granny decides to try to train her as a witch. This does not work. She has a different kind of magic inside of her. By the end of the book she is trying to become a student of the Unseen University of Ankh-Morpork to train as a wizard.
The book has interesting thoughts on traditional gender roles in fantasy books, and presents witchcraft in a novel way rather than the usual evil crones. In particular the way that Granny discusses her “headology” is fascinating. Esk is disappointed to learn that Granny almost never uses “real” magic as she considers it. Granny mostly uses a deep understanding of people’s perception of her to make it seem like she has. This is something like the placebo effect, and in Granny’s mind as much magic as what Esk wants to learn.