Ladyhawke by Joan Vinge, 252 pages

The spell of vengeance had been cast upon Etienne Navarre, Captain of the Guard, and the beautiful Lady Isabeau by the evil Bishop of Aquila. Etienne and Isabeau must wander the wilderness, always together yet always apart-she a hawk by day and restored to herself only with the setting of each day's sun; he a wolf by night, transformed once more into human form at break of each day's dawn. This eternal spell is their punishment for daring to love after the evil Bishop had already chosen Isabeau for his own. Then, suddenly, Etienne receives an unexpected sign of hope in the person of Phillipe, a young and cunning thief...and Navarre knows he must seize this fearful opportunity to free them from the Bishop's diabolical spell-or bring them death...