Nobody's Princess

This is Nobody's Princess by Esther Friesner. Really good.
Helen of Sparta is the future queen for Sparta. Unlike her sister, she wants to train with swords and go on brave and noble adventures. She finds a way to train with them dressing as a boy so the guards won't suspect. Her mother soon finds out she is training and doesn't mind. She told Helen stories of the time she was a huntress serving Artemis, the goddess of the Hunt.She teaches Helen how to hunt rabbits and deer, how she moves silently through the forest to outfox the fox and a out run the rabbit.Then the time comes when her sister goes to another place for marriage.Helen goes on the trip along with her brothers and sister and leave as quickly as possible. There, Helen had learned that the king had wanted to keep her there and will marry the youngest son of the king.They go when a messenger comes bringing news that a ferocious boar came through another place where Helen and her brother's uncle,Lord O (I can't remember some names) and their mother's sister rule. Helen stays there and learns of an amazing woman named Atalanta and she is training with the men for the hunt of the boar.Helen spies on her along with her horse and soon Atalanta offers horse lessons.(I will give you an update when I finish the book)