BOSTON -- For the first time, John F. Kennedy's secret love letters to a Swedish woman are going up for auction.

At 36, Kennedy was the dashing and ambitious senator who was already engaged to Jaqueline Bouvier. Gunilla Von Post was a beautiful, blue-blooded 21-year-old Swede. They met on the French Riviera in the summer of 1953, danced all night and parted with a passionate kiss.

So began a secret romance, documented in 14 letters and telegrams, from Kennedy to Gunilla Von Post. They have never been seen in public before.

She first revealed the affair in a 1997 book and in an interview on ABC's "20-20."

"His smile was contagious. It was really electrical between us," she said.

Only three weeks after the sparks flew in France, Kennedy married Bouvier in Newport, R.I.

But the next spring and summer, Kennedy wrote to Von Post to re-kindle their flame. "I thought I might get a boat and sail around the Mediterranean for two weeks -- with you as crew," he wrote.

"My heart, boom-boom-boom-boom. I was very happy to hear from him, but I said, 'He's a married man,'" Von Post said.

That summer their plans for a secret rendezvous fell apart when Kennedy injured his back. After two months in the hospital, he still pursued her.

"Under that beautiful, controlled face that still haunts me -- beats a warm heart," he wrote.

One year later, they met at a castle in Sweden.

"I borrowed him for a week -- a beautiful week that no one can take away from me," she said.

She kept Kennedy's letters locked up. On Monday, they'll be offered to the public on legendaryauctions.com.

Online bidding begins at $25,000. The letters will be auctioned off as a collection.

In Kennedy's final letter to Von Post, he wrote, "I just got word today that my wife and sister are coming here. It will all be complicated the way I feel now, my Swedish Flicka. All I have done is sit in the sun and look at the ocean and think of Gunilla."