It’s fair to say that Lady Glenconner has lived a life of extremes. From being a maid of honor at Queen Elizabeth’s coronation to a lifetime coping with a mentally unhinged husband whose idea of a honeymoon was taking her to a brothel in Paris to watch two strangers have sex. She captures all of this in her new memoir, Lady in Waiting, which will make you laugh and cry and gasp. She was lady-in-waiting—hence the title—to Princess Margaret, and these memories, having been written by one so close to the royal family, have sent shudders down the corridors of Buckingham Palace. There, silent discretion is paramount. And against that grain, Lady Glenconner kisses and tells, except they are not her kisses, but those of the people she spent her life attending.

Lady in Waiting discloses the secrets of polite British society and sheds light on the narcissism, drug abuse, infidelity, and betrayal that is concealed within these impossibly glamorous lives. But at the heart is loss, grief, stoicism, and love, as she navigates the painful narrative of her life, in which two of her sons die and one is crippled. “Before I started, I thought my life had been fairly humdrum,” Lady Glenconner explained to me from her cottage at Holkham, “but when I started to write, I began to realize that I did have a few stories.” Now 87, she has surprised everyone by sailing to the top of the U.K. best-seller list, and for good reason. “I’m amazed, because I’ve been invisible all my life,” Lady Glenconner said with a laugh. “With Colin [her husband] and Princess Margaret, one was completely invisible—you had to just follow along behind—and suddenly I’m visible, I’m there, I’m out. It’s the most extraordinary feeling!”

A mentally unhinged husband whose idea of a honeymoon was taking her to a brothel in Paris to watch two strangers have sex.

Born Lady Anne Coke, the eldest child of the fifth Earl of Leicester, she grew up at Holkham Hall, which she describes as so huge that when the footmen put raw eggs in a bain-marie as they walked from kitchen to nursery, they’d be boiled on arrival. Her childhood was spent up to no good in the corridors of stately homes with the young Princess Margaret. “Princess Elizabeth was much better behaved. ‘Please don’t do that, Margaret,’ or ‘You shouldn’t do that, Anne,’ she would scold us,” the author recalls. As a young woman, Lady Glenconner became the poster girl for the British aristocracy after serving as one of six maids of honor in white tulle at Queen Elizabeth’s coronation, in 1953, the most-watched event on television at that time.

On the occasion of her coronation, H.M. Queen Elizabeth II with her ladies-in-waiting, including Lady Glenconner (third from left), at Buckingham Palace, in 1953.

Lady Glenconner later became Princess Margaret’s lady-in-waiting, traveling the world with her to royal functions. They were Ronald and Nancy Reagan’s guests of honor; another time they stayed at a brothel in Swaziland, where Princess Margaret struggled to pin a medal on the king’s rather scant loincloth.

These anecdotes reveal a personal side to Princess Margaret, which has otherwise been inaccessible outside a close royal circle. Lady Glenconner tells me that part of the reason she wrote the book—other than being prompted by Helena Bonham Carter’s asking her for advice on how to play the princess in The Crown—was to set the record straight.

An honorable entourage: Princess Margaret (on bench) and her island community, photographed on Mustique in 1973.

Lady Glenconner’s second son, Henry, contracted H.I.V. in the 70s, and she describes how Princess Margaret would go to the Lighthouse, a hospice in West London that became a center for people with H.I.V. and AIDs, which she was patron of, long before Diana arrived with all the cameras. “No one saw Princess Margaret, but we’d both sit with the patients by their bedsides,” she explains. “She’d make them laugh and spend time with them, and they appreciated it. She never patronized or talked down to people.” Lady Glenconner stresses the grinding reality of what it means to be part of the royal family. “Meghan didn’t stay very long because she didn’t realize that to be a royal is jolly hard work and quite boring at times,” she told me. “It’s not all fun and glamour. A lot of it is behind the scenes, so it’s not supposed to be flashy.”

Princess Margaret struggled to pin a medal on the rather scant loincloth of the king of Swaziland.

Aged 23, Lady Glenconner married the Honorable Colin Tennant, later Lord Glenconner: narcissistic, charismatic, spoiled, and abusive. Aside from his inability to be faithful (he would randomly introduce illegitimate children into the family, only to decide that they weren’t to his taste and reject them), infantile tantrums formed an integral part of his everyday behavior. At one point, he threw himself on the floor of a plane aisle, screaming and shouting after being refused entry into first class. She later recounts an incident in Italy where he howled so loudly during a performance of Nabucco (because his West Indian manservant wasn’t there) that the conductor had to re-start the chorus of the Hebrew slaves from the top. So why did she put up with this? She stoically cites Tennant’s ironic dictum, “We were brought up not to throw in the towel, but to bite bullets and fold towels neatly.”

Wrapped in gold: Tennant with Lady Glenconner (left) and Bianca Jagger at Tennant’s 50th-birthday party on Mustique.

Did divorce really never cross her mind? “At one point, yes, when I was expecting my eldest son, Charlie,” she said, “I went back to my mother and told her what had been happening and that I couldn’t take it. She told me, ‘You go straight back,’ and I did, for 54 years.” Do you now think he suffered from a form of mental illness? “One of the doctors did say to me that he had incredibly thin skin. He just couldn’t take a lot of life—he couldn’t deal with things that you and I have to deal with.”

“Meghan didn’t stay very long because she didn’t realize that to be a royal is jolly hard work and quite boring at times.”

Out of everything, what was the hardest thing to write about, I asked her. “The boys,” she said after a pause, “it was very hard to write about the boys. After their deaths I needed to be strong for the other children, but as I wrote this book, I had to think about them, and it was very difficult.” Lady Glenconner’s oldest sons, Charlie and Henry, tragically died from hepatitis C and H.I.V./AIDS, respectively. Her account of their diagnoses and deaths make for two of the most moving chapters of the book. In fact, what starts as a book about royalty, aristocracy, and dysfunction becomes a meditation on loss and grief. When I ask Lady Glenconner’s longtime friend Rupert Everett for a few words, he writes back, “She has survived a lot of hardship. Two dead children is not something you easily come back from. I think she has a lot of deep sadness buried inside her.” But despite this, or perhaps because of it, “she is very resilient—a kind of fragile rock. She is what they used to call a ‘great lady.’”

Lady Glenconner marks the 60th anniversary of Queen Elizabeth II’s coronation, the first-ever coronation to be televised, at Westminster Abbey in 2013.

Lord Glenconner died with one last vicious sting left in his tail. Upon his death, he left his personal assets not to his infant grandson, recently left fatherless, nor to his wife, but to a former employee, a Saint Lucian boatman. How did Lady Glenconner cope with this final betrayal? “That was very difficult,” she says. “I wrote the book partly because I had no money, and, actually, I think I might make some!” So, has Lady Glenconner had the last laugh? “I have certainly had the last word, but it’s not a book of revenge in any way. I wrote about some of the bad behavior, not nearly all, but I never criticized him. It was important for me that my children would think that it was O.K.”

Eleanor Harmsworth is a freelance journalist based in London