The Wee Free Men (Discworld book 30), Terry Pratchett

A nightmarish danger threatens from the other side of reality . . .

Armed with only a frying pan and her common sense, young witch-to-be Tiffany Aching must defend her home against the monsters of Fairyland. Luckily she has some very unusual help: the local Nac Mac Feegle – aka the Wee Free Men – a clan of fierce, sheep-stealing, sword-wielding, six-inch-high blue men. Together they must face headless horsemen, ferocious grimhounds, terrifying dreams come true, and ultimately the sinister Queen of the Elves herself .

. . THE FIRST BOOK IN THE TIFFANY ACHING SEQUENCE