The Wee Free Men. A Tiffany Aching Novel (Discworld Novels)

Tiffany wants to be a witch when she grows up.
A proper one, with a pointy hat. And flying, she's always dreamed of flying (though it's cold up there, you have to wear really thick pants, two layers). But she's worried Tiffany isn't a very 'witchy' name. And a witch has always protected Tiffany's land, to stop the nightmares getting through.
Now the nightmares have taken her brother, and it's up to her to get him back. With a horde of unruly fairies at her disposal, Tiffany is not alone. And she is the twentieth granddaughter of her Granny Aching: shepherdess extraordinaire, and protector of the land.
Tiffany Aching. Now there's a rather good name for a witch.