In the Hand of The Goddess Tamora Pierce

From Tamora Pierce, the second book in the Song of the Lioness Quartet, honored with the Margaret A. Edwards Award. Alanna, disguised as a boy, becomes a squire to none other than the heir to the throne. 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 vicious 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 surrendering her dreams...