Princess Diana didn't like her hairstyle on her wedding day (exclusive)

If Princess Diana's former hairdresser Richard Dalton could say one thing to his most famous client today, he would keep it pretty simple, according to PEOPLE: “How would you like your hair done?”

August 31 marked the 27th anniversary of Princess Diana's death in a car crash in Paris. She was only 36 years old. Although Dalton had not worked with Diana for six years at the time of her death – he left her in 1991 after ten years together – he always kept her memory close to his heart. With the encouragement of his friend Renae Plant, curator of the Princess Diana Museum, Dalton has now shared those memories with Plant in his new book. It’s all about the hair – My decade with Diana (out now) and features never-before-told stories about the beloved royals.

“As a loyal subject of the Royal Family, I have never made anything public to protect them,” Dalton tells PEOPLE exclusively. “I am 76 years old, [the] same age as [King] Charles and I wanted to document my true story. [It’s] It's not a hairdresser's job to kiss and tell. I feel good now – 27 years have passed and nothing in my book is detrimental to Diana's character. Just wonderful memories.”

Dalton was responsible for Diana's hairstyle in the run-up to her wedding to Charles on July 29, 1981, but he was not the one who did her hair on the big day. That task was taken over by his colleague Kevin Shanley.

In the book, Dalton calls Diana's hairstyle on her wedding day “a disaster” – and the bride agreed with him.

“Diana expressed to me that she wished she could do the wedding again to 'get the hair right,'” Dalton tells PEOPLE. “However, she never mentioned any other regrets while I was doing her hair.”

Even though he sometimes had just 15 minutes to style the hair of the most famous woman in the world (and did so everywhere from the airplane bathroom to the kitchen sink in his own home!), “he never felt under pressure,” Dalton tells PEOPLE.

Princess Diana and Dalton loved to push the boundaries with their hairstyle. It had to be styled in every way imaginable, including wearing a variety of tiaras and hats on their heads to complete their look.

“I loved taking risks with Diana's hair and trying new things, like the orchards in Thailand,” he says, referring to his favorite hair moment ever for the princess. (Dalton says Diana liked it, too.)

At an event on February 4, 1988, Princess Diana wore a pink and purple Catherine Walker dress (which Dalton also wrote in his book was his favorite of hers). They were staying at the Imperial Hotel in Bangkok, Dalton writes, “and I literally ran around the hotel all day, stealing orchids from the flower arrangements to use as accessories in Diana's hair to match her dress,” the book says. “Whatever I saw, I took – and nobody stopped me. Eventually, I had all the colors I wanted to use – they represented Thailand and also matched her dress.”

For the occasion, Dalton styled Diana's hair into a chignon and pinned the flowers one by one on one side of her hair, he recalls.

“Fresh flowers had never been seen before, and she looked sensational that night – and she knew it,” Dalton tells PEOPLE.

During their decade together, Dalton saw Princess Diana at her best—her witty sense of humor, her devotion as a mother, her commitment to royal work (“Diana was very conscious that her hairstyle should not overshadow the charity she was representing and supporting that day,” Dalton says). But he also saw the former Princess of Wales at her worst, including moments of extremely low self-esteem during the low points of her marriage to the future King Charles, which eventually ended in a separation in 1992, a year after Dalton stopped working for her. (Charles and Diana’s divorce became final in 1996.)

“Some days she was happy, some days she was quiet, some days she was talkative, and some days she was sad,” Dalton says in the book. “I experienced all of that with Diana.”

He added that on her bad days, “sparks would fly out of the hairdryer,” he jokes in the book. “I got used to the stress.”

It was only when he ended his collaboration with her in 1991 that Dalton realized how much stress he was under, as he was responsible for the hair of the most photographed woman in the world.

The last time he did her hair, Dalton said she was “going through a really difficult period in her life with Charles and wanted fresh staff and realized I wanted my life back,” he tells PEOPLE. “She said, 'I think [you want] your freedom back' – and it was mutual. I only realised how much pressure there was when I left.”

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Much like the 2,700 items at the Princess Diana Museum, Plant tells PEOPLE that she sees preserving Dalton's treasure trove of memories as preserving history. Plant is also offering a special bonus: a free one-week membership to the Princess Diana Museum for anyone who purchases the book between August 31 and September 7.

“When these people die, their memories are lost forever,” she says, adding: “And we know that hairdressers are like best friends and hear everything.”

It’s all about the hair – My decade with Diana is out now.