Carolyn Bessette-Kennedy Book Dispels Rumor That a Pedicure Made Her Late for Final Flight with JFK Jr.

For decades, rumors have swirled that a spa appointment was the reason John F. Kennedy Jr., 38, his wife Carolyn Bessette-Kennedy, 33, and her sister Lauren, 34, were late to the airport the night of the fateful plane crash that killed all three on July 16, 1999. Now, a new biography is addressing it.

In Once Upon a Time: The Captivating Life of Carolyn Bessette-Kennedy, excerpted in this week’s issue of PEOPLE, author Elizabeth Beller writes that John, Carolyn and Lauren were all stuck in traffic on the way to the airport. John drove Lauren to the airport, where they met Carolyn who arrived separately. The plan was for John to drop Lauren off in Martha’s Vineyard before he and Carolyn flew to Hyannis Port for his cousin Rory’s wedding the next day.

"Tabloids put forth an alleged timeline in which Carolyn lingered for hours getting a pedicure,” Beller writes. “Yet an eyewitness report has her leaving there by 5 p.m. She went to Saks to buy a dress for the wedding and...after the purchase, the salesgirl wished Carolyn good luck. ‘Thanks,' Carolyn said, ‘I'm going to need it.’”

The weeks and months leading up to the flight had been tense for the couple. According to Beller, John and Carolyn had begun marriage counseling that spring and were both under tremendous stress, including the impending death of John's beloved cousin, Anthony Radziwill, pressures at John’s political magazine George and the difficulty Carolyn was having with the photographers constantly camped outside their door.

“They were both very sympathetic to what the other one was going through, but also getting a little impatient,” Beller tells PEOPLE. “Sort of, ‘When will things be okay at George? When will we stop having to have every night be about going out to promote the magazine or look for advertisers?' And [John] felt like, ‘When are you going to be able to handle the press, because it's not going to go away?’”

While rumors of affairs later surfaced after Calvin Klein model Michael Bergin, whom Carolyn had dated in the early '90s, wrote a 2005 book The Other Man, in which he claimed that he continued to be romantically involved with Carolyn during her marriage to John, Beller says the fact that Carolyn joined John on the trip to Rory's wedding was a sign they were trying to make it work.

“I know [Carolyn] didn't want to go to the wedding. I think she had been to so many events, she just needed a break,” Beller says. “And I think he understood that. But he had said he would go, and he knew that if he went alone, then the press would be all over them even more. And they had a fight about that.”

“But the bottom line is, she did end up deciding to go,” Beller continues. “She went with him. She made that commitment to say, ‘I will go with you.’”

Beller adds their story is also a reminder about the other side of celebrity.

“This is something to take away — how hard it is to be in the public eye…and hoping that people are a little gentler on people who are in the public eye,” she says. “Things may look a certain way from the outside, and they're not that way on the inside.”

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Once Upon a Time: The Captivating Life of Carolyn Bessette-Kennedy is on sale May 21 and available for preorder now, wherever books are sold.