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Former President John F. Kennedy carried on an 18-month affair with a White House intern, according to a new book by the alleged mistress -- now a 69-year-old grandmother and retired New York City church administrator.

The book, "Once Upon a Secret: My Affair with President John F. Kennedy and Its Aftermath," out Wednesday, was bought by the New York Post at a Manhattan bookstore.

Mimi Alford writes that she met Kennedy in the summer of 1962 as a 19-year-old debutante from New Jersey whose finishing-school polish and blueblood connections had landed her a job in the White House press office.

Four days into her internship, she claims she was invited by an aide to go for a midday swim in the White House pool, where the 45-year-old president swam daily to ease chronic back pain. Kennedy floated up to her and introduced herself, she writes.

Later that day, Alford was invited by Dave Powers, the president's "first friend," to a White House party.

She claims her affair with Kennedy began when the president invited her for a personal tour and then took her to "Mrs. Kennedy's room."

"I was in shock," she writes of her post coital thoughts. "He, on the other hand, was matter-of-fact, and acted as if what had just occurred was the most natural thing in the world."

Their trysts took on a routine, she writes. Alford says she would swim with the president at noon or at the end of the workday, race back to her desk and wait for a call to visit him upstairs.

She never once ran into Jackie Kennedy during these flings and admits to not feeling guilty.

Alford claims that their affair continued even after she returned to college at Wheaton, an all-girls school in Massachusetts. The president used the pseudonym "Michael Carter," when he called her and arranged trips to visit him in Washington when his wife was away.

She recalls visiting with Kennedy as he was embroiled in one of the most defining moments of his presidency, the Cuban Missile Crisis in 1962.

"I'd rather my children red than dead," she claims he told her during the nuclear standoff with the Soviets.

The book also includes salacious details of wild partying, including a Hollywood bash at Bing Crosby's desert ranch, where he coerced her to try the drug poppers.

He did not try the drug himself, she said.

About to be married to her college sweetheart, Tony Fahnestock, Alford describes meeting Kennedy for the last time at The Carlyle hotel in Manhattan on Nov. 15, 1963, just seven days before his assassination in Dallas.

"He took me in his arms for a long embrace and said, 'I wish you were coming with me to Texas.'

And then he added, 'I'll call you when I get back.' I was overcome with sudden sadness.

'Remember, Mr. President, I'm getting married,'" she writes.

"'I know that,' he said, and shrugged. 'But I'll call you anyway.' "

Click here to read more at the New York Post.