The author told CBS 'This Morning' Co-Anchor Mark McEwen that his research for the book completely changed his opinion of the heir to the British throne.
"We thought of him as a stiff, unloving royal," Andersen said of Prince Charles. "Certainly that was the image in the press that he had, much like his mother, the queen. I think this was a turning point in his life."
Andersen said conversations with the staff at the Paris hospital where the critically injured Diana died provided a glimpse of Prince Charles' first reaction to seeing the dead princess.
"It was a stunner," he said. "The chief nurse there, the hospital in Paris, led Charles and Diana's two sisters into the room where Diana was lying in a casket. They had prepared the body for viewing.
"He looked at her first, and, for the first time, froze by the casket and reeled back in horror. They thought he was going to faint, but he managed to pull himself together enough to comfort Diana's two sisters."
Andersen said the staff who told him the stories did so through tears: "Many became very emotional months later, recalling the scene."
He said Paul Berell, Diana's butler and a close confidant, said he was surprised Charles was so devastated and destroyed by her death.
On the other hand, the queen's first thought when she heard the news was of jewelry.
"She calls the British embassy in Paris and asks if Diana was wearing any royal jewels and, if so, she wanted them back immediately," Andersen said.
"This sent the British consul general running to the hospital. According to the nurse on the scene, he came into the room in an agitated state and said, 'The queen wants to know where the jewelry is. The queen wants to know where are the jewels.' That was her first thought, not whether Diana had gotten proper medical treatment or not whether Diana suffered."
Andersen says the queen did not consider the princess a member of the royal family and had no idea how well-loved the princess was. As a result, she was pitted against her son in planning for the mourning and funeral.
"[The queen] did not want Charles to claim the body and escort it back to London," he said. "Shdid not want the flags to fly at half-mast. She did not want a public funeral. Every step of the way, Charles fought against his mother.
"She did not have any plans to speak to her subjects in the days leading up to the funeral. The U.K. was crying out for her to come out and say something. Her son finally delivered an ultimatum. He said, 'I'll do it.' He was backed by Tony Blair, the prime minister. He knew the British people were in a state of tremendous grief and pain over Diana's death."
Andersen also said he discovered that the body of the princess, who had been known for fashion and her sense of style, was returned home to London in a borrowed dress.
Asked about the source of some of his information, particularly when quoting from a phone call between Charles and his longtime friend and confidante, Camilla Parker-Bowles, Andersen said the substance of such conversations was related to him by friends.
"[Parker-Bowles] had two friends that confirmed that, as did Charles. They have very close people around them they have talked to for many years and they tell everything to."
A palace spokesman calls the claims "completely inaccurate and without any foundation whatsoever."
She says the allegations are deeply hurtful to the royal family, especially coming so close to the first anniversary of Diana's death.
Andersen says the the book was based on interviews with palace sources, hospital workers, and staff at the Ritz Hotel in Paris.
"We treat this book with the contempt it deserves," a palace spokesman said. "The [book's reported] reaction [to Diana's death] by the queen is particularly inaccurate."