Prince Charles Exploited Diana's Love of Fashion to Make Her Seem 'Frivolous,' New Book Claims

"Learning that her clothes were used against her, was really sad to me," fashion journalist Elizabeth Holmes tells PEOPLE

Princess Diana delighted in fashion in a way that no royal has since. But her husband Prince Charles used her love of style to undermine her, Elizabeth Holmes' new book, HRH: So Many Thoughts on Royal Style, claims.

"The attention Diana received was surprising at first to the royal family, and then a source of resentment," Holmes, whose book published on Tuesday, tells PEOPLE. "For Charles, you can tell in his public remarks, fashion is one of the things he sought to use against her. [He used Diana's] interest in fashion, and the interest in her fashion, to paint her as shallow or frivolous."

"What an outdated and unfortunate line of critique, especially because these women use their clothing so savvily," she continues. "Learning that, and learning that her clothes were used against her, was really sad to me."

From left: Princess Diana and Prince Charles in 1987. Georges De Keerle/Getty

Holmes, who spent more than 10 years as a business and fashion journalist for The Wall Street Journal, gained a massive following for her Instagram series, "So Many Thoughts," in which she explored royal fashion. After interviewing a number of fashion experts and designers, some of whom have worked intimately with the royal family, Holmes expanded her analysis of the "power" of royal fashion to create HRH, a deep dive into the branding and fashion journeys of Queen Elizabeth, Princess Diana, Kate Middleton, the Duchess of Cambridge, and Meghan Markle, the Duchess of Sussex.

Princess Diana and Prince Charles on their wedding day. Terry Fincher/Princess Diana Archive/Getty Images

While Holmes revels in all royal fashion, she says that Diana's outfits are some of her favorites. The Princess of Wales wasn't afraid to embrace trends — or to put her emotions on display, Holmes explains.

"What I came to really understand and appreciate was the ways in which she delighted in fashion, and introduced emotion into the equation," says Holmes. "She had these very distinct phases in her dressing and they track with the tumultuous personal life she was experiencing."

Tumultuous it was. Diana was only 20 years old when she married the 32-year-old Prince of Wales in 1981. From the beginning, their relationship was fraught with tension. The young princess struggled to manage both the public scrutiny and the expectations of royal life (she suffered from an eating disorder and depression). In 1991, the royal couple, who share sons Prince William, now 38, and Prince Harry, now 36, separated. Three years later, Charles admitted he'd committed adultery during his marriage to Diana during a television interview. (Prince Charles resumed a relationship with his ex-girlfriend (and current wife) Camilla Parker-Bowles.) Their divorce was finalized in August 1996, and Diana, who was already devoting herself to humanitarian causes during their separation, honed in on her work to help lift up others. Tragically, she died just a year later when her car crashed in a tunnel in Paris.

Throughout it all, Diana's clothing choices were at the forefront — and Charles took notice.

During a visit to the Gulf States in 1989, Charles insulted Diana in front of their royal hosts, according to The Diana Chronicles by Tina Brown. An emir turned to Diana to ask what she had planned during the official visit — Diana was scheduled to visit a "clinic for immigrant women" and tour a center for children with mental disabilities — when Charles answered for her.

Princess Diana and Prince Charles. Jayne Fincher/Princess Diana Archive/Getty

"Shopping, isn't it, darling?" Charles said, according to The Diana Chronicles. (The scene is also cited by Holmes.)

"The words dropped into the marble stillness like bricks into plate glass," said Diana's private secretary Patrick Jephson in Brown's book. "[Diana] colored, mumbled something inaudible, and lapsed into silence."

While Charles felt unseated by the attention his young wife was getting, Diana only wanted to be seen by him.

"No matter what you were making for her," said designer Jasper Conrad, according to HRH, "the question was always, 'Will my husband think I am sexy in this?'"

"At her heart, I think she wanted Charles' his attention. And so, so many of her outfits, especially in the late '80s, seem to scream, 'Look at me,'" Holmes explains. "The most famous example is her revenge dress that she wore the night the documentary aired in which Charles admitted that he had been unfaithful."

Holmes says that Diana could have responded to the public humiliation in a variety of ways, including staying at home. But she didn't.

"Instead, [Diana] pulls out this off the shoulder, above the knee, fantastic little black dress and just sort of wowed," says Holmes. "The pictures from there ended up running alongside the news of his affair. To insert herself in the narrative like that, to reclaim her voice by not saying a word, just stepping out of the car in a fantastic dress, she knew how to use fashion to get attention."

The "Revenge Dress". Getty

Diana may have wanted her husband's attention, but she also loved fashion for its own sake, especially after she received a royal makeover with the help of British Vogue editors. Her goal was to fulfill the public's expectation of a princess, from her voluminous wedding dress to her glamorous daytime wear, Holmes explains.

"She knew that time spent with her, anticipating her, remembering her, was important," Jephson told Holmes in the book. "It was a comfort, it was a pleasure, it was a break from what might be a very difficult existence."

He added: "It was also a show of respect. People who met Diana usually got up early, did their hair and put on their nicest clothes... The least she could do was make an effort as well."

In the early years of Diana's marriage, she wore long hemlines and high collars to appear older, but gained confidence as the years went by. Then "as her marriage hit the rocks, her clothing choices grew even more daring," says Holmes. "And that's some of my favorite fashion in the book."

For Holmes, no one in the royal family has ever been as bold in their fashion choices as Diana.

"You look at what she was wearing and how outrageous it was, and you can just tell her spirit's there to make those choices," she says. "I mean, we just don't see fashion-forward royals these days."

Some of Diana's most style memorable moments came at the end of her marriage and the beginning of her new life, the author explains.

"It was sort of in the middle of her divorce, and afterward, you can tell [Diana] began dressing for what she wanted to wear. She knew what looked good on her body. She knew what photographed well," Holmes says. "There's a spread in the book about some Versace outfits. They're those bright, solid colors, very clean in spirit, and they fit like a glove. They're exciting but still modest and very much appropriate. She just looked fantastic."

Princess Diana in Versace in 1996. Getty

The princess touring minefields.

Then, a couple of months before her death, the princess emptied part of her closet. She auctioned off 79 of her most iconic dresses for charity.

"[Diana] was shedding her skin after the divorce," says Holmes. "She wanted to be seen as a workhorse, not a clothes horse."

Like her daughters-in-law, Kate Middleton and Meghan Markle, Diana knew when to dress more casually in order to get the most attention for the causes she cared about.

Holmes adds: "Some of the most iconic images is Diana traversing a landmine, in a button-down, and khakis, and loafers. Fashion was not part of the equation at all. But there's real power in that, too."