She always called him “Mr. President” — not Jack. He refused to kiss her on the lips when they made love. But Mimi Alford, a White House intern from New Jersey, was smitten nonetheless. She was in the midst of an 18-month affair with the most powerful man in the world, sharing not only John F. Kennedy’s bed but also some of his darkest and most intimate moments. In her explosive new tell-all, “Once Upon a Secret: My Affair with President John F. Kennedy and Its Aftermath,” Alford, now a 69-year-old grandmother and retired New York City church administrator, sets...