A Hat Full of Sky (Discworld: Tiffany Aching, Bk. 2)

Witch-in-training Tiffany Aching hadn't expected magic to involve chores and ill-tempered nanny goats! But as Tiffany pursues her calling, a sinister monster pursues Tiffany, and neither Mistress Weatherwax (the greatest witch in the world) nor the six-inch-high Wee Free Men (the greatest thieves in the world) can defeat it. When the monster strikes, Tiffany will have to save herself - if she can be saved at all!