It is difficult to know whether the elegiac mood I felt while reading The Shepherd's Crown was due to the book itself or to the fact that the fifth Tiffany Aching novel (and forty-first Discworld novel) was in fact the late Sir Terry Pratchett's final work.
The Shepherd's Crown focuses on the young witch Tiffany Aching as she comes fully to find her place both in the non-hierarchy of the witches' world, in the land of her birth (the Chalk), and in her own life. She finds herself pulled between two steadings, the districts for which, as a witch, she is responsible for doing "what needs to be done" — whether visiting the old and sick, birthing babies, or protecting the inhabitants from supernatural invasion. And, as the book begins, a supernatural invasion does in fact loom: Nightshade, Queen of the Faeries (whom a nine-year-old Tiffany defeated in the first book in the series) finds that the boundaries between her world and Tiffany's are weak, and she is planning large-scale revenge. Discworld faeries have much more kinship to the Celtic sidhe than to the cute winged creatures of most children's books or than to Tolkien's aristocratic elves: they are (literally) glamorous, pitiless creatures who take delight in mayhem ranging from spoiling beer and stealing sheep to kidnap, torture, and murder.