Wintersmith : (Discworld Novel 35)

When the Spirit of Winter takes a fancy to young witch Tiffany Aching, he wants her to stay in his gleaming, frozen world. For ever. It will take all the young witch’s skill and cunning, as well as help from the legendary Granny Weatherwax and the irrepressible Wee Free Men, to survive until Spring.
Because if Tiffany doesn’t make it to Spring –– Spring won’t come. THE THIRD BOOK IN THE TIFFANY ACHING SEQUENCE