Less than a month after her death, Morton has revealed what has been long suspected - that the Princess played an active role in his book. "To all intents and purposes it was her autobiography," he says. According to his latest revelations, she gave six lengthy interviews to him as well as informal conversations. She also read the manuscript of the book and made detailed changes and supplied photographs and captions.
In past few weeks, two features have characterised the aftermath of Diana's death: the view that she was a defenceless victim of the press and promises from the media to avoid similar intrusions on privacy. Morton's revelations have blown both these notions away. They have brought us down to earth, to a more realistic image of Diana and a depressingly familiar view of what can be expected of the press.
Diana - Her True Story was the first book that revealed details of Diana's eating disorders, her suicide attempts, and the continuing affair between her husband and Camilla Parker Bowles. It was explosive, showing quite how badly the fairy-tale marriage had come unstuck. Morton himself was roundly attacked and at first disbelieved.
But the book was phenomenonally successful, selling five million copies worldwide. And now more than 100,000 copies of the new edition, Diana, Her True Story - In Her Own Words have been printed to cope with the huge demand for the book which is expected to be released on 6 October.
This edition will make more millions for Morton even though it offers no fresh revelations - except to claim that around 18,000 words of the book are the Princess speaking directly. Well, sort of. Morton admits that he never met the Princess face to face and conducted the interviews by proxy.
Diana was fingered as the source at the time of the book's initial publication though she denied involvement. But her tacit approval was made clear. The Princess visited Carolyn Bartholomew, her friend who had told Morton that Diana had suffered from bulimia. And she told journalists, "I did not co-operate with this book in any way," but added the pointed aside, "What my family and friends do is a matter for them.''
Morton chose to tell the story through the third person and quoted close friends such as James Gilbey and Bartholomew. Now he says that many of the most sensational disclosures actually came from the Princess herself - that she was, in his terms, the "Deep Throat" of his book. "She was like a prisoner wanting to smuggle a story out."
His latest editing proves fascinating for Di-addicts to analyse - she changed a description of Prince Charles from "the man she longed to marry" to "the man she was in love with". Referring to the couple meeting in 1977 (when Prince Charles went out with Diana's sister) the text originally said: "Diana has since told friends, `I kept out of the way. I remember being podgy, no make-up, an unsmart lady but I made a lot of noise and he liked that.' " In fact she told that directly to Morton.
But the book does more than allow us to play psychological games. It has altered the climate, putting things back to normal. The near deification that followed Diana's death has stifled any comment about her skilful and almost unerring instinct for PR.
Earl Spencer may have described his sister as "the most hunted person of the modern age" but it is unfair to her, and less than she deserves, to show her as just the passive, saintly victim of the media. Diana's appeal was precisely that she was not a saint but a woman - and one who could be remarkably astute about her appeal.
And in the battle of PR she beat her husband - and the Royal Family - hands down. She refused to play the royal wives' game of put up and shut up. The two most explosive revelations about The Firm both came from her - Morton's book and her Panorama interview.
Diana's decision to grant Martin Bashir the 1995 interview confirmed many of Morton's original claims but also added her own twist - remember "There were three of us in this marriage so it was a bit crowded." Or, "I'd like to be a queen of people's hearts ... but I don't see myself being Queen of this country."
But is it right for Morton to reissue the book by putting words directly in Diana's mouth? He says "it is important as a matter of historical record that - to quote her brother Earl Spencer - `she is allowed to sing openly'." On the contrary, it's ghoulish. It is the ultimate invasion of privacy. The Princess went to lengths to supply Morton with information but not to meet him. Even in her Panorama interview, when she again confirmed many of the claims in the book, she distanced herself from it. To reveal what many of us expected to be true, and so cash in on the current Diana publishing sensation, is to betray a confidence she obviously wanted and felt she could trust.
Latterly, of course, newspapers have argued and pondered over the use of photographs invading people's privacy. Yet the right to have the privacy of one's thoughts and words respected seems to have passed The Times by. Yesterday it dedicated its first three pages to Diana's words.
"It is right that Diana herself has the chance to speak," Andrew Morton said yesterday. "The Establishment tried to deny it to her in life. I do not think they can deny it her in death." Get real. It's not a case of denial or suppression. In death, Diana hasn't a choice whether Morton chooses to reveal her words or not. Now that's what I call hunted.