A design Jacqueline Kennedy Onassis sent to designer Bill Hamilton at Caroline Herrera.

She drew a sketch herself that Hamilton then turned into a design. The final product was a brown velvet suit, which made her very excited. Onassis was often very hands-on during the process.

"We would have lunch at my desk and she would pull out a peanut butter and jelly sandwich that she carried in her bag and have coffee," Hamilton said.

Fourteen photos taken by Bob Davidoff, a long-time photographer for the Kennedy family, are also part of the auction.

An image of Jacqueline Kennedy Onassis by Bob Davidoff up for auction.

A letter Jacqueline Kennedy Onassis sent to designer Bill Hamilton at Caroline Herrera.

A letter Jacqueline Kennedy Onassis sent to designer Bill Hamilton at Caroline Herrera.

An image of Jacqueline Kennedy Onassis by Bob Davidoff up for auction.

For her home, she worked with interior designer Richard Keith Langham.

"She had a hand in everything there," Langham said. "Whether she would be dictating exactly what sort of velvet she wanted on a chair, exactly what sort of cording she wanted to trim a pillow."

Most of the letters, many written in her signature blue stationary, were actually thank you notes to both men, letting them know how happy she was with the results.

In one note to Keith Langham, Onassis writes: "I wasn't mad about the black chairs. Too heavy, I'm afraid -- so do keep looking."

More than a 1,000 people are expected to take part this weekend. Each lot, some containing two or three letters, is said to be worth $800 to $1,200.