This is the third Discworld book, and the first in the witches series. 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 newborn baby who has just been born in a rural village. The child is supposed to be the seventh son of a seventh son, but turns out to be a daughter. 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 apply to train to be a wizard in the Unseen University of Ankh-Morpork.
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.