The Wee Free Men: (Discworld Novel 30).
EVERY LAND NEEDS ITS OWN WITCH . . .
Up on the Wold, there's a monster in the river and a headless horseman in the drive. And now Granny Aching has gone, there's only young Tiffany Aching left to guard the boundaries. To stop ... things getting through.
It's her land. Her duty.
But it's amazing how useful a horde of unruly pictsies can be — as long as they are pointed in the right direction and can stop fighting each other first . . .
FROM MASTER STORYTELLER TERRY PRATCHETT