The Centaur's Daughter by Ellen Jensen Abbott

Rather a nice YA fantasy. I believe it is the second book in the series, but from the backstory filled in in this one, I am not certain how the first one played. It sounds like it was the girl's father's story, but that wouldn't exactly be true to the genre.

Abisina's father was a shape-shifter, his other form, a centaur. She thinks she is a normal human, but under the stress of trying to rescue a friend, she changes also. For some reason which is never entirely clear, she tries to hide this fact from the community which owes its existence to her father, the shapeshifter.

Most of the YA themes are present, and fairly well done. I think it is often easier for writers to deal with that sort of thing in a fantasy context.