The Wolf Hunt by Gillian Bradshaw
When Marie Penthievre of Chalendrey is abducted from her Norman priory and taken to Brittany's court, she vows never to dishonour her family's ties by marrying a Breton brute. There is only one man who might change her mind: Tiarnan of Talensac, a handsome, skilled, and noble knight indeed. But Tiarnan does not love her, and when he marries a slip of a girl instead, Marie vows to become a nun as soon as she is able. But Tiarnan has a secret, and that slip of a girl betrays him once she learns of it. When Tiarnan disappears and is presumed dead, his widow marries his one-time rival and assumes the title to his land, which steadily begins to decline under her unskilled, merciless rule. Marie knows something is wrong and only she is clear-headed enough to rescue Tiarnan and return him to his rightful status. But can she do so before it is too late?