The jacket copy makes your mouth water with tantalizing promises of wealth, glamour and power. Even the title bespeaks upper-class gentility: American Lady: The Life of Susan Mary Alsop. This biography is the story of “the second lady of Camelot.” Many of us thought in the Kennedy administration that title belonged to the vice president’s wife, Lady Bird Johnson. While we’re never told who bestowed the honorific on Mrs. Alsop, we are assured that she is “an American aristocrat,” who “reigned over Georgetown society for four decades, her house a gathering place for everyone of importance, including John F. Kennedy, Katharine Graham and Robert McNamara.”

As someone who lives in Georgetown and enjoys reading about the myths of Camelot and American aristocrats, I could hardly wait to gobble up this book.

Perhaps the author, a member of the Conseil d’Etat, the highest administrative court in France, and once a diplomat, could not shake her silk stocking background long enough to probe beneath the surface. Or maybe she drew too close to Mrs. Alsop’s family, who gave her access to letters, papers and diaries that she barely quotes. Perhaps it was the author’s intercontinental collaboration with her sister, whom she credits with helping her complete the book. Then again, it might be the translation from French to English that makes this book (at 256 pages) read like biography lite.

“Slim” is the word de Margerie uses to describe Mrs. Alsop, an understatement for the stick-thin woman I met in Washington, D.C. (we went to the same Georgetown hairdresser). At 5’7” she was almost skeletal and appeared to weigh no more than 95 pounds, with blue-veined skin tissued over protruding bones. Even her son, William S. Patten, described his mother as “anorexic.” Yet her biographer chooses a polite characterization that is fathoms from the destructive disease of anorexia.

“Slim” seems to be the operative word for this book, whose euphemisms keep readers removed from knowing the woman whose life intersected with many charismatic figures of her era — Franklin Delano Roosevelt, Winston Churchill, Greta Garbo, Noel Coward, Edith Wharton, Brooke Astor, Ho Chi Minh, the Duke and Duchess of Windsor, and President and Mrs. John F. Kennedy.

Susan Mary Jay Patten Alsop (1918-2004) was the great-great-great-granddaughter of John Jay, one of the Founding Fathers, who signed the Treaty of Paris in 1783 and became the first chief justice of the United States. Born into a revered American family, she declared she saw “no future in being an ordinary person.” She didn’t even like associating with ordinary people. According to her biographer, “She had always preferred lords to cowboys.”

Born in Rome, Susan Mary, as she was called, grew up in South America, traveled in Europe, lived in Washington and New York, and summered in Maine. She graduated from the Foxcroft School, took a few courses at Barnard College and married at the age of 21 after meeting Bill Patten (Harvard, Class of 1932), nine years her senior. He was severely asthmatic, like her father, declared 4F and unfit for military service. So his mother-in-law, Mrs. Jay, got him hired by the U.S. State Department to work for Sumner Wells, undersecretary to Cordell Hull. Patten’s first assignment was Paris, where he and Susan Mary lived from 1945 to 1960, when he died of emphysema.

Susan Mary soon fell out of love with her “sickly but sweet” husband and became besotted with their close friend, Duff Cooper, the British ambassador to France (1890-1954), the great love of her life. Her biographer maintains that Bill Patten “never showed signs of torment or bother” over his wife’s affair, and Susan Mary “was convinced he did not know.” She gave birth to Cooper’s son in 1946 and kept the paternity secret from her husband. She finally told her son when he was 47 who his real father was.

Three months after Bill Patten Sr. died, his Harvard roommate, Joseph Alsop, proposed. A powerful (and pompous) political columnist, he was syndicated in over 300 newspapers. After the election of John F. Kennedy, Alsop decided he needed a hostess to entertain the president and first daddy so he wrote to Susan Mary, saying they could become a part of history if she married him. He admitted he was homosexual, said he did not expect her to be in love with him and that she could take a lover at any time. Reckoning that her son and daughter needed a stepfather and she needed a place in Washington society, Susan Mary accepted. The marriage did not last, but their friendship endured to the end of their lives.

Following her separation from Alsop, she asked him for permission to continue using his name. At the age of 56 she became a writer and published her first book, To Marietta from Paris: 1945-1960, a compilation of her letters to her best friend, Marietta Peabody Fitzgerald Tree. She published three more books and then became a contributing editor to Architectural Digest for many years.

Toward the end of her life she was plagued by near blindness, drug addiction and alcoholism. She died at the age of 86 in the Georgetown house she had inherited from her mother. Her obituaries barely mentioned her career as a writer, celebrating her instead as a “socialite,” “hostess” and “Washington doyenne.” Most assuredly she would have been pleased by the social plaudits because her first priority as an American lady was maintaining her place in society. Readers accustomed to hearty biography will go away hungry from this little morsel, feeling deprived of the banquet promised on the book jacket.

Kitty Kelley, who has written seven biographies, will publish Capturing Camelot (Thomas Dunne Books) on November 13, 2012.