The Woman Who Rides Like a Man (Song of the Lioness, Book #3) by Tamora Pierce
From Tamora Pierce, the third book in the Song of the Lioness Quartet, honored with the Margaret A. Edwards Award.
Newly knighted, Alanna of Trebond seeks adventure in the vast desert of Tortall. Captured by fierce desert dwellers, she is forced to prove herself in a duel to the death—either she will be killed or she will be inducted into the tribe. Although she triumphs, dire challenges lie ahead. As her mysterious fate would have it, Alanna soon becomes the tribe's first female shaman—despite the desert dwellers’ grave fear of the foreign woman warrior. Alanna must fight to change the ancient tribal customs of the desert tribes—for their sake and for the sake of all Tortall.