LONDON — A girl meets a prince. The prince needs a wife. The glass slipper fits: she is young, dewy, inexperienced, eager — swept up and blinded by the romance of it all. He proposes within months. Their wedding is watched by 750 million people worldwide and features at least five clergymen, 3,500 guests and one spectacularly frothy pearl-encrusted wedding dress whose 25-foot train has to be stuffed into the bridal carriage, where it threatens to suffocate the bride.

That was 30 years and a lifetime ago, and there was to be no happily ever after for Prince Charles and Diana, the Princess of Wales. (He turned into something of a frog; she grew, in life and in death, to be more Evita than Cinderella.) In retrospect, it was probably not very helpful that the groom’s sometime mistress and future second wife was among the wedding guests....