I Shall Wear Midnight (Discworld book #38), Terry Pratchett

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 . . .