First in the Song of the Lioness series. Alanna so wants to become a knight that she convinces her twin brother to trade places when their father sends them off for training. It was not a hard sell since he wanted to be a sorcerer (all noble girls go to a convent for this training and noble boys go for knight training). She assumes the disguise of a boy and he a girl. Her path is hard-- the day is not long enough to get all the training and homework done, a bully picks on her, and fighting needs extra effort as she is one of the smallest there. She makes friends with Prince Jonathan and his friends. Her magic, learned from a local healer, saves the prince from the sweating fever. She and the prince defeat the immortals using their combined magic.
Sequel: In the Hand of the Goddess