Beneath the Moon — Patricia Robins

Jennifer Ames was not beautiful but she had a certain something that kept Peter Barrington in willing thraldom for ten years. One day they hoped to marry, and to speed the happy day, Jenny takes a job as governess to Lord Barclay's twin children, Michael and Marie. When Jenny arrives at the Worcestershire manor house, she find herself engulfed in mystery. Why does the tall, good-looking Derek both fascinate and frighten her?