A Hat Full of Sky. Discworld Novel 32

Something is coming after Tiffany...

Tiffany Aching is ready to begin her apprenticeship in magic. She expects spells and magic – not chores and ill-tempered goats! Surely there must be more to witchcraft than this! What Tiffany doesn't know is that an insidious, disembodied creature is pursuing her. This time, neither Mistress Weatherwax (the greatest witch in the world) nor the fierce, six-inch-high Wee Free Men can protect her. In the end, it will take all of Tiffany's inner strength to save herself...

if it can be done at all.