Every famous person has one other individual behind his or her back, and not at the center stage. For the icon Jacquelyn Kennedy Onassis, it is her sister Lee Radziwill. But it is not to say that the latter was in the shadow for she was a person of her own.

In a rare occurrence, Radziwill, an American socialite, shares a glimpse of her life with her famous sister while promoting her book, "Lee." The book was given an exclusive preview in the new issue of People.

Of all the people who'd understand Radziwill for having a prominent figure in their life, Prince Phillip, Queen Elizabeth's husband, would be it. Lee recalls and writes in her book, the Prince having spoken to her when she and her sister went to Buckingham Palace way back in 1961, saying, "You're just like me - you have to walk three steps behind."

The book is a full of fond memories, too personal even, and it shows with the handwritten caption. Even when promoting her book, she still guards per privacy.

Her friend, magazine editor Richard David Story reveals with People, "She doesn't really like to promote herself like that. This book was something very special to her. But she doesn't want to talk about herself." 

The sisters were also very close, according to a very close observer of the sisters' lives, Clint Hill, who was the First Lady's Secret Service agent, People adds. "Mrs. Kennedy relied on Lee. "Anytime there was some kind of an event that was crucial, Lee usually was present." 

When asked about sibling rivalry, Hill adds, "I don't think they we were really competitive. I do think that Lee was somewhat jealous of Mrs. Kennedy because of her position. She was received as the wife of the President and Lee was kind of like her lady-in-waiting when we were overseas."

Lee and Jackie were the only two children of Janet and John Bouvier III.

"Lee" will be released on Dec. but can be pre-ordered via Assouline.