“Once Upon a Secret” includes a couple of truly vile episodes in which the president humiliated Mimi by telling her to service other men sexually. But the first part of the book mostly presents her as a willing, star-struck participant who appealed to the president’s snobbery. “He just couldn’t resist a girl with a little bit of Social Register in her background,” she writes.

It is only when she begins a courtship with a college boy (“Williams! How could you?” teases the Harvard-educated president) that her meltdown starts. Ms. Alford, put to the test, was willing to pre-cheat on her future husband, Tony Fahnestock. And he married her even after he learned about this betrayal. They stayed married (apparently without discussing President Kennedy) for more than 20 years.

From the wedding onward “Once Upon a Secret” becomes increasingly crazy and sad. Ms. Alford gives up on the idea that one day she and Tony will laugh about her youthful indiscretion. They settle into an angry, joyless union. She starts reading squishy self-help books and parroting their platitudes. She takes up marathon running and claims to have been “assigned” by the New York Road Runners to share a hotel room in London with a fellow athlete with whom she had been flirting. She is identified as a Kennedy girlfriend in 2003 and claims to have enjoyed being trapped in her New York apartment while reporters staked out the building. “I caught up on my reading and knitting,” she writes.

What now? Serenity of course. Ms. Alford claims to be completely purged of guilt, grief and baggage by the cleansing process of acknowledging past mistakes. And she describes a happy new marriage, albeit in the strangest terms. (“What could be better, I thought, than a man whose enthusiasm for Brussels sprouts matched mine?”) She describes working for a church without being religious. She writes about weekly budget-balancing with her husband as if it were more fun than lolling around in the president’s bathtub. And, most astoundingly, she ends the book with an inspirational account of how she and her husband visited the Kennedys’ graves at Arlington National Cemetery.

Thinking of the president lying beside the first lady did make her feel “like an intruder,” she admits. So the etiquette lessons of her cotillion days at least taught her something. But she also mouths the words “Thank you” at the gravesite, because she believes President Kennedy gave her a terrible secret that became a great blessing that became a miraculous redeeming force, a gift that now warrants public celebrating. It took an awful lot of Brussels sprouts to give her the clean conscience she boasts today.