Some 14 years worth of correspondence with an Irish priest, the Rev. Joseph Leonard, are set to be sold at auction in June.
In the letters, Jacqueline Kennedy compares the then-congressman's ambition to that of Macbeth. She also compared him to her father, John Vernou Bouvier.
"He's like my father in a way -- loves the chase and is bored with the conquest -- and once married needs proof he's still attractive, so flirts with other women and resents you. I saw how that nearly killed Mummy," she wrote.
After the president's assassination in 1963, she told Leonard she felt "bitter against God."
"I have to think there is a God -- or I have no hope of finding Jack again," she wrote. "God will have a bit of explaining to do to me if I ever see Him."
The letters are expected to fetch more than $1 million at auction in Dublin.