Tehanu — Ursula K. Le Guin

Once she'd been a priestess, quest-companion to a powerful mage, a student of high magic. Then she gave it all up to be a farmer's wife on Gont, content to lead a simple life. But Tenar was not born to live her days in peace, away from great events. A dying wizard and an abused child were the first to call her back to the path she had abandoned. For the end of the adventure beckoned and Tenar would be there along with the dragons, mages, and the young king himself to share in the unforgettable fate of the kingdom known as Earthsea.