The Stone Fey — Robin McKinley

Maddy has been roaming the hills of Damar with her sheep since she was a girl. The Hills hold everything she desires: her family; her beloved dog, Aerlich - and soon, her fiancé, Donal, who has been away for a year. But one evening a lamb is lost. And when Maddy returns to the Hills to find it, she discovers something else the Hills possess - something that will change her forever...