Terry Pratchett 
I Shall Wear Midnight 
(Discworld Novel 38)

As the witch of the Chalk, Tiffany Aching performs the distinctly unglamorous work of caring for the needy. But someone or something is inciting fear, generating dark thoughts and angry murmurs against witches. Tiffany must find the source of unrest and defeat the evil at its root. Aided by the tiny-but-tough Wee Free Men, Tiffany faces a dire challenge, for if she falls, the whole Chalk falls with her . . .THE FOURTH BOOK IN THE TIFFANY ACHING SEQUENCE