The Tower of Ravens: Book One of Rhiannon's Ride

One-Horn’s daughter is not like the others of her kind. Born of a human father, she lacks the horns so prized by her people and is scorned even by her own mother. Her only chance for escape is to capture one of the legendary flying horses and ride it to freedom.
 
So this strange, feral girl begins a dangerous journey of love, death, and betrayal that will earn her a new name: Rhiannon, the rider no one can catch.
 
Rescued and taken to the home of Lewen, a young man just beginning to understand his own magical potential, Rhiannon is fascinated by the human world and by Lewen. Together they travel through a land where the dead walk and ghosts haunt the living, a place where Rhiannon encounters dark forces that endanger all of Eileanan. But to save the land, she must convince Lewen and the other apprentice-witches to trust the word of a wild half-human girl.