In the Hand of the Goddess
From Tamora Pierce, the second book in the Song of the Lioness Quartet, honored with the Margaret A. Edwards Award.
Still disguised as a boy, Alanna becomes a squire to none other than the prince of the realm. Prince Jonathan is not only Alanna's liege lord, he is also her best friend—and one of the few who knows the secret of her true identity. But when a mysterious sorcerer threatens the prince's life, it will take all of Alanna's skill, strength, and magical power to protect him—even at the risk of revealing who she really is...