Alanna: The First Adventure by Tamora Pierce

Alan of Trebond, the best warrior in the palace, is harboring a big secret: he is really a she - Alanna. But when her prince is felled by an illness, ...more Call it fate, call it intuition, or just call it common sense, but somehow young Alanna knows she isn't meant to become some proper lady cloistered in a convent. Instead, she wants to be a great warrior maiden--a female knight. But in the land of Tortall, women aren't allowed to train as warriors. So Alanna finds a way to switch places with her twin, Thom, and take his place as a knight in training at the palace of King Roald. Disguising herself as a boy, Alanna begins her training as a page in the royal court. Soon, she is garnering the admiration of all around her, including the crown prince, with her strong work ethic and her thirst for knowledge. But all the while, she is haunted by the recurring vision of a black stone city that emanates evil... somehow she knows it is her fate to purge that place of its wickedness. But how will she find it? And can she fulfill her destiny while keeping her gender a secret?