A newly released memoir is shedding fresh light on how Jackie Kennedy really felt about her second husband.

Architect John Carl "Jack" Warnecke — who was romantically involved with the former first lady from 1964 to 1966 — writes that she confessed regret over her union with Aristotle Onassis. In one passage he recalls: "She unhesitatingly expressed frustration that this one room was all she was allowed to decorate [and] told me she believed that her marriage to Onassis was the biggest mistake she had ever made."

Warnecke died in 2010. His book "Camelot's Architect: The Untold Story of Jackie Kennedy, Lafayette Square and a Secret Romance" hits shelves on Nov. 10 and an excerpt surfaced this week.

Friends long after the romance

According to the memoir the two stayed close for years after they split. Once Onassis passed their bond deepened again.

"She delighted in advising me in what she perceived to be my serious relationships with other women. Or possibly, she was testing me," Warnecke wrote.

He described the closeness as feeling like an "exciting secret affair" yet insisted "nothing physical" happened between them during that later stretch.

Jackie stayed married to Onassis until he died in 1975. She lost her battle with cancer in 1994.