Author Wendy Holden exclusively tells PEOPLE she was surprised at how “extreme” real royal life was behind the palace gates

Princess Diana and author Wendy Holden. PHOTO: TIM GRAHAM PHOTO LIBRARY VIA GETTY; LAURIE FLETCHER

Like millions of other people around the world, author Wendy Holden says she was “transfixed” when she watched the wedding of Lady Diana Spencer to the then-Prince Charles.

Now, following in-depth research into the late Princess Diana’s life as she drew her own picture of the late royal for a new novelization of her extraordinary life, Holden says she has been surprised at how “extreme” real royal life actually was behind the palace gates.

“It was a collision between different aspirations and ideas. The royal family wanted Charles to get married because he was 30, and they wanted a particular kind of girl,” Holden tells PEOPLE. “She needed to be young, she needed to be aristocratic, and she needed to be without a past. It was a very pragmatic decision. She was practically the only person who was left. He had had so many girlfriends by then.”

Speaking at the launch of The Princess in London, Holden adds, “From her point of view it was love — she thought he was going to sweep her away and be a knight on a white charger, everything she always wanted. I hadn’t appreciated the extent to which what we saw the day before the wedding wasn’t just a little bit different from the reality but the actual complete reverse.”

Princess Diana and Prince Charles (with Sarah, Duchess of York) in 1983.PRINCESS DIANA ARCHIVE/GETTY

“How extreme it was, how dramatic it was and how different it was,” the author continued. “That was the surprise to me.”

In The Princess, Holden tells the story of Diana’s childhood, her time sharing an apartment with girlfriends in London, her entry into royal life and her ultimately unhappy marriage, much through the eyes of a fictional friend. Following The Governess — the story of the teacher and childhood confidante of the late Queen Elizabeth — and The Duchess on Wallis Simpson, it is the latest in a series of historical fiction trilogy in which Holden has conjured up novels based on the lives of women in the royal family.

For authors like Holden, Diana — who died following a Paris car crash in August 1997 — has moved from being a contemporary figure to a historical one. She says there has been a “generational” change that allows her, as a writer of historical fiction, to inhabit that space now. And for her book, Holden consulted many biographies, including Andrew Morton’s Diana: Her True Story (which used many of Diana’s own words), to gain a sense of one of the most iconic women of the 20^th century.

Princess Diana and Wendy Holden.TIM GRAHAM PHOTO LIBRARY VIA GETTY; LAURIE FLETCHER

So what would she be like today, as her sons Prince William and Prince Harry navigate their own challenges – most notably their estrangement? Those arguments, of course, might never have happened if she were alive, Holden points out.

The author adds: “She had a good sense of humor so maybe she’d have been able to dial down these entrenched positions. She would have put family above all and relationships above all and tried not to have them not take up these extreme positions.”

Prince William, Princess Diana and Prince Harry in 1995.ANWAR HUSSEIN/GETTY

The Princess is published in the U.S. by Berkley in paperback and the U.K. by Welbeck in hardback.

Wendy Holden’s book based on Princess Diana.WELBECK