Sir Philip Sassoon (1888-1939), a glamorous and well-known figure in Britain for the first four decades of the twentieth century, was the most eligible bachelor and the greatest host of his time. He attained prominence in the art world, high society, and politics. In contrast, his sister Sybil (1894-1989) lived a much more private life. Yet she was fascinating in her own right, marrying into the grandest level of the English aristocracy, restoring...