Story highlights Mimi Alford is now a 69-year-old grandmother

The author says her affair with John F. Kennedy lasted 18 months

"I'm not a virgin anymore," Alford recounts after first encounter with Kennedy

The alleged affair took place in the summer of 1962, according to her book

The author of a new tell-all book claims she lost her virginity to President John F. Kennedy when she was a 19-year-old White House intern, and that the affair lasted 18 months.

The book, "Once Upon a Secret: My Affair with President John F. Kennedy and Its Aftermath," will be available Wednesday, but CNN was able to buy a copy Monday.

The alleged affair began in the summer of 1962, while Mimi Alford worked in the White House press office. She met Kennedy four days into her internship, and was invited for a personal tour of the residence a day later by the commander-in-chief after being served daiquiris, she says in Chapter 4 of the book.

"Slowly, he unbuttoned the top of my shirtdress and touched my breasts," Alford said in the book. "Then he reached up between my legs and started to pull off my underwear. I finished unbuttoning my shirtdress and let it fall off my shoulders."

"After he finished, he hitched up his pants and smiled at me" pointing her to the bathroom.

"I was in shock," she writes. "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."

The White House arranged for a car for her, she said.

On the ride home, it "kept echoing in my head: I'm not a virgin anymore."

"In an era when women in the workplace were still considered 'girls,' Mimi was literally a girl herself -- naive, innocent, emotionally unprepared for the thrill that came when the president's charisma and power were turned on her full-force," according to the website of the book's publisher, Random House. "She was also unprepared for the feelings of isolation that would follow as she fell into the double life of a college student who was also the secret lover of the most powerful man in the world."

Fifty years later, Alford is now a 69-year-old grandmother and retired New York City church administrator, the Post reported.

Following Kennedy's assassination, Alford "grieved in private, locked her secret away and tried to start her life anew, only to find that her past would cast a long shadow -- and ultimately destroy her relationship with the man she married," Random House said.

In 2003, a Kennedy biographer mentioned "a tall, slender, beautiful 19-year-old college sophomore and White House intern, who worked in the press office" when referencing one of the president's affairs, the publisher's website said. "The disclosure set off a tabloid frenzy and soon exposed Mimi and the secret that she had kept for 41 years."

The publisher describes the book as "a new and personal depiction of one of our most iconic leaders and a powerful, moving story of a woman coming to terms with her past and moving out of the shadows to reclaim the truth."