His eulogy at Diana’s funeral was electrifying. As her brother, Earl Spencer, stared down at the Royal Family, vowing to protect Diana’s sons and allow their “souls to sing openly”, the unthinkable happened. The vast crowds of mourners outside Westminster Abbey began to applaud. Slowly, the ripple of applause moved forwards towards the Abbey doors and then to the congregation inside. Spencer’s words had ignited the fire of indignation about how Diana had been treated in life – and perhaps in death too. In the front rows, most of the Royal family sat in stony silence. But William and Harry gingerly applauded their uncle.

Now, with the 30th anniversary of the Princess’s death on the horizon, he has written what promises to be the definitive account of his sister’s all too short life: a young woman he brilliantly defined as “unique, complex, extraordinary and irreplaceable”.

Charles Spencer is an established historian, author and journalist. He does not dabble in the sensational and he has clearly become frustrated by the many myths surrounding Diana that have somehow become accepted as truths. Besieged with requests for interviews about the Princess, he has decided to speak once again on her behalf in what he says will be an openly positive way. No one is better placed to do this than he, and it promises to be a fascinating read – as well as an important historical document which will separate fact from fiction.

Some of it is almost certainly going to be uncomfortable reading for the King. Earl Spencer witnessed the turmoil of the troubled “fairytale” marriage at first hand. I don’t think he will deliberately set out to hurt or embarrass his ex brother-in-law, or indeed, Camilla. But the truth from his perspective will inevitably be awkward and potentially damaging.

His own relationship with his sister was not always smooth. They sometimes squabbled, as many siblings do. But ever since that funeral oration, Spencer has been fiercely protective of Diana’s memory and, indeed, of her burial place on his estate at Althorp.

And quietly, in the background, he has been true to his promise to stay close to William and Harry. Only last month Harry took his family to stay at Althorp, where they visited Diana‘s grave. Perhaps the Earl was the first to know about Harry and Meghan’s decision to move back to the UK? Perhaps they’ve been using him as a sounding board?

There is so much that Spencer can put on record. He has already voiced his visceral anger about how the disgraced BBC reporter, Martin Bashir, persuaded his sister to speak out on Panorama. And there’s bound to be more on that. But his account of exactly what happened during the critical week after Diana’s death, when the Palace disastrously lost control of the narrative, will be particularly significant.

His book should also enlighten a generation who cannot remember the phenomenon that was Diana, Princess of Wales and sadly believe everything they saw in the Netflix series . As the Earl puts it, she was a dynamic character who gave life “a hell of a go, and left the world a far better place”. He hopes his book will help young people “get” who she really was. It should be one hell of a read.