Terry Pratchett 
A Hat Full of Sky 
(Discworld Novel 32)

THE SECOND BOOK IN THE TIFFANY ACHING SERIES
Something is coming after Tiffany. . .
Tiffany Aching is ready to begin her apprenticeship in magic, but life isn»t exactly what she thought it would be.
She expects spells and magic – not chores and ill-tempered goats! Surely there must be more to witchcraft than this?
And Tiffany will find that she needs her magic more than ever, to fight off the insidious, disembodied creature that 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. Will she succeed?