Four or five years ago, Carly Simon set out to write a book about her relationships with the women in her life: her close friends, her two older sisters and her glamorous, complicated mother.

“Girls were my first love,” Ms. Simon said. “They were the first people I was really close to until I met my first boyfriend. And my house was overloaded with girls.”

But as she continued to work on the memoir, one friendship in particular called to her: an unlikely decade of intimacy with former first lady Jacqueline Kennedy Onassis, whom she met on the Vineyard in 1983.

“I started out writing about strong women in my life and then — it’s sort of like you reduce a soup, you reduce a sauce. It just seemed to be there was more that she required, a different place,” Ms. Simon said.

The result is her new book, Touched By the Sun: My Friendship with Jackie. Ms. Simon will appear at a book signing to promote the new work at the Bunch of Grapes bookstore in Vineyard Haven this Saturday, Dec. 21 at 4 p.m.

The two women were introduced by John Kennedy Jr. — Heidi Wild

The two met when John F. Kennedy Jr. introduced them at the Ocean Club restaurant in Vineyard Haven shortly after Mrs. Onassis bought land in Aquinnah. Mrs. Onassis, then an editor at Doubleday, soon attempted to persuade Ms. Simon to write a memoir. Daunted (then at least) at the idea of unloading details of her personal life, Ms. Simon instead wrote a series of books for children illustrated by Island artist Margot Datz. Over the course of that collaboration, she and Mrs. Onassis became friends, sharing dinners at each other’s homes, meeting for lunch dates and movies in New York, gossiping about mutual acquaintances and exchanging a large volume of written correspondence.

When Ms. Simon spent six days in a rehab program in the midst of a crumbling marriage to her second husband, it was Mrs. Onassis she spoke to each day from the facility.

“It was Jackie that I called because she was so interested. For some reason she was so interested in my life,” Ms. Simon recalled. “And of course I never could understand that. I kept trying to understand why is she so interested in me?”

They shared much in common. Each was a mother. Each had emerged from difficult marriages. Each was an icon of her generation. Both were acquainted with the exhausting distortions that come with a lifetime in the spotlight. But Ms. Simon posits that their differences were the real basis of Mrs. Onassis’s interest. Ms. Simon was a bohemian, an independent artist who crescendoed into adulthood during the 1970s. Mrs. Onassis was an idol of elegance and taste who came of age in the 1950s. Ms. Simon never depended on a husband to support her. Mrs. Onassis almost always did.

“We were of a different generation. I was doing things that women of her age growing up in the 40s and 50s wouldn’t have had the chance to do,” Ms. Simon said. “I was very lucky. I came into writing songs just at the time it was okay to be a songwriter.”

Ms. Simon describes Mrs. Onassis’s artistic inclinations: her literary career, her drawings, her poetry and of course, her style. They danced together in Mrs. Onassis’s living room, donned costume wigs for an evening out, spent afternoons dissecting the lyrics of Ms. Simon’s ex-husband James Taylor’s newest songs, trying to decipher his meaning.

“If she hadn’t grown up to be Jackie Kennedy Onassis, I think she still would have had more of an artistic life,” Ms. Simon said.

The relationship was sometimes fraught. Ms. Simon describes Mrs. Onassis’s abiding hesitance to reveal too much. There is also a painful moment recounted when the two women were sitting in a movie theatre and Ms. Simon asked Mrs. Onassis whether she had seen a new movie about President Kennedy’s assassination. Ms. Simon writes that Mrs. Onassis “reacted as if she had been attacked.” She said, “Oh no, Carly, no. No, no.”

“I was a little bit on my guard when I was with her,” Ms. Simon said. “I could only tell when I came home how tense my neck was.”

Though her friendships, daughterhood and sisterhood with other women were largely taken out of the book, Ms. Simon’s view of Mrs. Onassis seemingly expanded to envelop them all. Ms. Simon describes the former first lady as a giggling accomplice, a sister, an ally, and even a mother figure, all in turns. Ms. Simon said she believed Mrs. Onassis would like the result.

“I think that she would’ve been reticent at first,” Ms. Simon said. “She would have said, oh Carly you can write about so many more interesting people than me. I would have said, no, Jackie, you’re the most interesting person in the world.”

She said she wanted to share a side of Mrs. Onassis many others did not see: a rebel, a comic, an artist. She recounts a moment when, sitting together on Mrs. Onassis’s bed, Mrs. Onassis said, “You called me by my name. You see through me.”

“I will remember forever how it felt,” Ms. Simon writes.