Heather Terrell, who writes under the pen name Marie Benedict, has written about novelist Agatha Christie in The Mystery of Mrs. Christie, and in Lady Clementine, she looked back on the life of Winston Churchill’s wife, Clementine Churchill. Now, in her historical novel The Mitford Affair, she has turned her attention to three English sisters—Unity, Nancy and Diana Mitford—with the rise of Nazi Germany as a backdrop.