As a subject, the princess proves to be something she never was in life: obliging. Beautiful, bad-tempered, scandal-prone, she makes for unfailingly good copy, and heaps of it. “Everyone seems to have met her at least once or twice, even those who did their best to avoid her,” Brown writes. “She shows up without warning, popping her head around the door of every other memoir, biography and diary written in the second half of the 20th century” — usually to insult her hostess or use someone’s hand as an ashtray.

Image Craig Brown

But, for a time, her charms were considerable. “Little hot looking pretty girl,” according to Ralph Ellison. Picasso desperately wanted to marry her. Peter Sellers would have settled for an affair. John Fowles publicly fantasized about abducting her and keeping her as a prisoner.

When the playwright Alan Bennett visited a friend, the television interviewer Russell Harty, on his deathbed, Harty requested the tracheotomy tube be removed. He just had to report that Margaret had inquired about his health — twice.

The princess could, her father said, “charm the pearl out of an oyster.” Her interests, however, ran more to sadistic parlor games (she makes an indelible appearance in Edward St. Aubyn’s Patrick Melrose novels). She was aflame with snobbery and, as she grew older, addicted to bullying and one-upmanship. She would boast about her royal status to her children, and insist lovers address her as Your Royal Highness. When needing a rest, she was known to commandeer the Queen Mother’s wheelchair.

“Disobedience is my joy,” she supposedly told Jean Cocteau. But it was more than that; it was her identity. It was the opinion of Gore Vidal, one of her more loyal friends, that since the queen was the source of national honor and duty, it fell to the princess to be the evil sister, the source of “creative malice.” (Of Vidal, the princess once said dryly: “The trouble with Gore is that he wants my sister’s job.”)