Harry’s will not by any means be the first publication from within the firm. His father, the Prince of Wales, is a prolific author on gardens, art, architecture and the environment, most recently Climate Change: A Ladybird Expert Book. Princess Michael of Kent has published seven books including the Anjou Trilogy, three historical novels set in 15th-century France. The Earl of Snowdon is the author of three books on furniture and Sarah, Duchess of York, has written her own kiss and don’t quite tell Finding Sarah: A Duchess’s Journey, as well as the much more edifying cookery book Dining with the Duchess.

There is even a precursor to a royal memoir, of sorts. In an era in which the oration and the epistle were prized literary genres, Queen Elizabeth I’s command of these forms made her a significant author. She also wrote in occasional verse. The other monarch memoirist was Queen Victoria who began a daily journal in 1832 and remained, for all her time on the throne, a prolific diarist and correspondent.

It is estimated that Victoria wrote 60 million words during her lifetime and her 122 volumes, edited and expurgated after the Queen’s death by her daughter Princess Beatrice, can still be read, albeit only by someone with a lot of time on their hands. It might be an idea for someone to play the Princess Beatrice role with Harry. On the instruction of her mother, Beatrice took out anything from the diaries which she thought might upset the royal family. This is not likely to be the approach adopted by J R Moehringer, the ghostwriter with whom Harry has already been collaborating for a year.

The template for Harry’s book might not be The Tender Bar, Moehringer’s account of his own troubled childhood, but it might well be his work with Andre Agassi on Open. This is a chronicle of a young man forced into duty he loathed by an over-bearing father. The most notorious line from Open could be rewritten substituting the word royalty for tennis: “I play tennis for a living even though I hate tennis, hate it with a dark and secret passion and always have.”

But perhaps the most obvious model for the book will be Andrew Morton’s 1992 book Diana: Her True Story. That, after all, is the source of the story Harry has to tell and therein lies the main problem with the idea of doing the book at all. Harry has taken a lot of flak already for the way he courts the media on his own terms. It is not credible, runs the argument, to complain about press intrusion and then do a staged conversation with Oprah Winfrey or a televised bus ride with James Corden, sign deals with Spotify and Netflix, let alone write a memoir. Yet this isn’t really problem. Press intrusion on the royal family did get out of hand. Of course Harry has a point. What reader could contemplate the fate of his mother and not concede that much?

The real problem with Harry’s memoir is that it will be organised around a contradiction that the named author cannot escape. In the end, Andre Agassi could get away from his father by marrying Steffi Graf and settling down to educational philanthropy in Las Vegas.

Harry does not have that option because he is not, as Agassi was, merely a celebrity who craved a quiet life. He is a royal, which is a status by birth, and his only claim on our attention, and the only reason for the vast advance, is that he will tell the inside story. The revelation of Prince Harry is a palace revolution or it is nothing. He wants to write “not as the prince I was born but as the man I have become”. It is only the former that keeps anyone even remotely interested in the latter.

Harry clearly has a lot more to say. The global sales will prove there are plenty of people prepared to listen. But whether it is wise to keep speaking is quite another matter. The first Queen Elizabeth put it well in a poem called Doubt of Future Foes: “For falsehood now doth flow, and subjects’ faith doth ebb/ Which should not be, if reason ruled or wisdom weaved the web”.