Swan Song: Diana, My Sister, by Charles Spencer was released on Tuesday after damaging snippets serialised in a British newspaper attracted sensational headlines around the world.
Spencer argued with the then-Prince Charles over palace plans for Prince William and Prince Harry, then 15 and 12, to walk behind their mother's coffin during her 1997 funeral procession.
Spencer says he told Charles that Diana, who was killed in a car crash in Paris at the age of 36, would not have wanted her boys to undergo the ordeal.
He claims the prince "went ballistic" during the argument, and told him: "Rest assured, we'll forget her soon enough".
Spencer also accuses Charles of having "contempt" for his ex-wife and of sounding "elated" after her death.
After excerpts of the book were published last week, Buckingham Palace responded with a strongly-worded statement that's rare by the measured standards of the royal household.
"His Majesty is mindful that the pain of fraternal grief can cloud reason, affect judgment and color memory in ways others do not recognise, even many years after such a loss," it said.
Spencer has stood by his claims, insisting to the BBC that he is telling the truth.
On the royal rebuttal, he asked: "Is he furious? Is he furious or is he embarrassed?"
The war of words drew renewed attention to Charles and Diana's stormy history.
Charles, then 32, married 20-year-old Lady Diana Spencer in a lavish ceremony at St Paul's Cathedral in 1981. The couple divorced in 1996, and a decade later Charles married Camilla Parker Bowles, now Queen Camilla.
Charles Spencer, the 9th Earl Spencer, has long clashed with the royals over their treatment of his sister.
Tensions between the palace and the Spencer family following her death were dramatised in the film The Queen and the Netflix series The Crown.
In a searing eulogy at his sister's funeral, Spencer vowed that her "blood family" would carry on her "imaginative and loving" legacy in raising her two sons. The speech was widely considered a swipe at the royal family.
The royals have spent decades putting that period of crisis behind them.
In his book, Spencer also criticised the royals for what he described as a "lack of obvious care" for the then 12-year-old Harry.
He recounted how "tiny, and slight, and bereft" his nephew looked after the funeral.
"He needed Diana's hugs, of course, and they would forever be denied him," he wrote.
"But instead of calm and nurture, he was allowed to spin out of control."
Some critics have questioned why Spencer felt the need to revisit the pain and trauma nearly 30 years on.
The aristocrat said he wanted to write down his own thoughts and memories about Diana, "once and for all, rather than having the discomfort of again reading the opinions of others, many of which are based on untruths".
Penguin Press said the book will be published globally and translated into 12 languages.
