Shirley Jones may be best known as the wholesome matriarch of "The Partridge Family," but according to her new book, "Shirley Jones: A Memoir," her life off camera was marked by a tumultuous marriage, infidelity and ultimately divorce.

In the memoir, which hit bookshelves July 23, Jones reveals that her nearly two-decades-long marriage to actor Jack Cassidy was anything but innocent. Likely to the surprise of her fans, she details a threesome she and her former husband once had with a dancer she refers to as Jean.

"If Jack hadn’t fully come to terms with my sexual boundaries before, after that night in Las Vegas with Jean, he now understood conclusively that threesomes were just not my thing. From then on, I assumed that he went his own way, sexually speaking, and, I guess, had threesomes with other women instead. I was still madly in love with him, and my awareness of his many infidelities, in whatever permutations he chose, didn’t tarnish my love for him. Nor did our threesome with Jean."

Jones admits that she knowingly turned a blind eye to his many indiscretions.

"Off camera, in the real world, the Swinging Seventies were in full bloom, and Jack was determined to explore every aspect of the new sexual freedom. He wanted it all: swinging, pornography, drugs, group sex. I carried on just looking away and ignoring his infidelities."

She also reveals that she and Cassidy briefly separated for an eight month period before reconciling, much to the dismay of a psychiatrist she had been seeing at a time.

"He was never able to be alone, so now he wanted me to take him back. I didn’t have to think too long about my answer. We had three children together, he was their father, and despite everything I still loved him. The psychiatrist I was then seeing told me that I was making a big mistake by taking him back: “You are never going to change that man. Don’t go back into this marriage thinking you can.” He was right, of course, but the Shirley Jones who had fallen so deeply in love with Jack Cassidy so many years ago wasn’t prepared to relinquish her dream man. And I didn’t."

Eventually, however, she did. The pair split in 1974 following Cassidy's 1973 diagnosis as manic depressive and bipolar.

"The truth about the man I loved, and about our marriage, was penetrating my consciousness at last, so that I finally started to come to terms with the harsh reality that my life with Jack, my dream man, my white knight, my sexual Svengali, might well be ending forever."

This isn't the first time Jones has opened up about her relationship with Cassidy, who died in a fire in 1976. She told newspapers in 1983 that if he were alive, she doubted that she would have ever married her second husband Marty Ingels, to whom she is still married.

"He wanted to come back (to me) right up to the day he died," she said. "And as I realized later, I wanted him. That's the terrible part. Much as I love Marty and have a wonderful relationship –- I'd say this with Marty sitting here –- I'm not sure if Jack were alive I'd be married to Marty."

Click through the slideshow below to see what other celebrities have opened up about divorce in their memoirs.

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  • sTORI Telling by Tori Spelling

    Tori Spelling and ex-husband Charlie Shanian <a href="http://www.people.com/people/article/0,,1117511,00.html" target="_hplink">split in October 2005</a>, merely 15 months after their $1-million wedding. In 2008's <em><a href="http://www.amazon.com/sTORI-Telling-Tori-Spelling/dp/1416950737" target="_hplink">sTORI Telling</a></em>, she discusses the demise of their brief union, including details of her affair with her current husband and former costar Dean McDermott. <blockquote>"It needed to end, but it could have happened in a nicer way. I wish I'd been able to communicate with Charlie from the very beginning of our relationship. And I should have handled things differently in the end. He's a sensitive, kind person. I know he would have listened. If I'd let our differences reveal themselves earlier...well, we might never have gotten married."</blockquote>

  • My Remarkable Journey by Larry King

    In Larry King's 2009 memoir, <em><a href="http://www.amazon.com/My-Remarkable-Journey-Larry-King/dp/B004JU1S58/ref=ntt_at_ep_dpi_1" target="_hplink">My Remarkable Journey</a></em>, the former TV host discusses his <a href="http://www.huffingtonpost.com/2010/12/21/larry-king-famed-host-ope_n_799802.html" target="_hplink">eight marriages and seven divorces</a> casually--as if they were nothing more than small steps on his road to success. Of his second wife, Annette Kaye, he writes: <blockquote>"After she got divorced, she told me she's done so because of me and insisted that I marry her. She could be very controlling, and I was never good at saying no. We were married at Broward City Hall. But I was young, and I wasn't going to be boxed in. I never lived with her. I can't remember seeing her much after the wedding--maybe a couple of times. Divorce papers were filed as a matter of course."</blockquote>

  • What Falls Away by Mia Farrow

    Actress and activist Mia Farrow married her first husband Frank Sinatra when she was 21 and he was 50. She discusses their divorce in her 1997 memoir, <em><a href="http://www.amazon.com/What-Falls-Away-Mia-Farrow/dp/0553763342/ref=sr_1_1?s=books&ie=UTF8&qid=1298420868&sr=1-1" target="_hplink">What Falls Away</a></em>, revealing that she was shocked to be served with divorce papers on the set of <em>Rosemary's Baby</em>, her breakthrough film.<blockquote> "Then, without warning, on an afternoon in November, Frank's lawyer, Mickey Rudin, appeared on our set carrying a brown envelope. He pulled out documents just long enough to see they were made out in my name: they were an official application for a divorce from Frank Sinatra. I remember the unprofessional look of surprise as Mr. Rudin realized I had not expected his visit, nor did I know anything about the papers he carried. This was the first mention of divorce. I held myself together and signed all the papers without reading them. If Frank wanted a divorce, then the marriage was over."</blockquote>

  • Through the Storm: A Real Story of Fame and Family in a Tabloid World by Lynne Spears

    Eight years after Britney Spears' parents' divorced--and two years after her mom, Lynne Spears, <a href="http://www.amazon.com/Through-Storm-Story-Family-Tabloid/dp/1595551565 " target="_hplink">wrote a memoir</a> chronicling her ex's alcoholism her failed marriage--<a href="http://www.huffingtonpost.com/2010/11/16/britney-spears-parents-ba_n_783952.html" target="_hplink">the pair reconciled</a>. In her book, released in September of 2008, she wrote: <blockquote>"When we spoke our vows to each other, those tender, simple words, I could never have imagined for one split second that someday everything would unravel, that in a short period of time I would live each day in uncertainty, never knowing if this was the day the man I loved would act like my husband again. And for the better part of the next twenty-four years, he wouldn't act like the man I married."</blockquote>

  • Official Book Club Selection: A Memoir According to Kathy Griffin

    Kathy Griffin, who divorced her ex-husband Matt Moline in May 2006, revealed that he had <a href="http://articles.cnn.com/2006-07-10/entertainment/griffin.lkl_1_d-list-matt-moline-kathy-griffin?_s=PM:SHOWBIZ" target="_hplink">stolen over $72,000</a> from her over the course of their marriage, which prompted the split. In her 2009 <a href="http://www.amazon.com/Official-Book-Club-Selection-According/dp/0345518519" target="_hplink">memoir</a>, she wrote: <blockquote> "Looking back on it all, I really had been a complete pussy about realizing it was over, even though in the back of my mind the trust that should exist between a married couple was 100 percent shot," she wrote in her memoir, published in September 2009. "I wish I had one of those Stella Got Her Groove Back moments where I turned to him and said, "Get out!" But in the end it was Matt who had to hit me over the head with the fact that we were through."</blockquote>

  • Charlie & Me by Harriet Bronson

    In <em><a href="http://www.amazon.com/Charlie-and-Me-ebook/dp/B004EPYUNU" target="_hplink">Charlie & Me</a></em>, talk show host and author Harriet Bronson--Charles Bronson's first wife--details the demise of her marriage and her ex's affair with a younger co-star, British actress Jill Ireland, whom he later married. In her memoir, published in December 2010, she wrote: <blockquote>"Charlie couldn't understand the extent of my anger. How could I explain it to him? For eighteen years I'd loved him, trusted him, believed in his truthfulness and his integrity, and now he'd betrayed me. The man living in the house with me wasn't even Charlie anymore. He was somebody else, and I didn't like him or want him. I felt overwhelmed and victimized by circumstances beyond my control, circumstances that the perpetrators would not even own up to."</blockquote>

  • Happens Every Day: An All-Too-True Story by Isabel Gillies

    Former <em>Law & Order: SVU</em> actress Isabel Gilles quit her acting career to follow her husband, a poet and professor, to his new teaching job at Oberlin College. In her 2009 memoir <em><a href="http://www.amazon.com/Happens-Every-Day-All-Too-True-Story/dp/1439110077" target="_hplink">Happens Every Day: An All-True Story</a></em>, her best-selling memoir, Gilles reveals that he abruptly left her for another professor, turning her seemingly idyllic life upside down. <blockquote>"My beloved husband whom I was so deeply in love with was going away from me and I couldn't stop him. Our children couldn't stop him. Reason couldn't stop him. Money couldn't stop him. Our parents couldn't stop him. Nothing could stop him, and it made me want to tear my hair out and cry until the world ended. But the world wasn't going to end. In fact, it kept going at an alarmingly fast pace."</blockquote>