The fifth and final book in the Tiffany Aching series - five Discworld novels by master-storyteller Terry Pratchett.
Tiffany...

Koom Valley, the ancient battle where the trolls ambushed the dwarfs, or the dwarfs ambushed the trolls, was a long time...

There's trouble on the Aching farm - a monster in the river, a headless horseman in the driveway and nightmare spreading down...