In Swan Song: Diana, My Sister, Charles Spencer alleges that he 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 colour 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 or is he embarrassed?"
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 older sister.
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.
At the time, many in Britain and beyond felt Queen Elizabeth II and her kin were too slow to acknowledge the global outpouring of grief over Diana's death.
After days of news stories that pitted one Charles against another, the book landed in shops on Tuesday with a new offering of salacious allegations.
Spencer says his sister tried to break off her engagement after discovering Prince Charles had bought a bracelet for Camilla.
She also tried to back out of the wedding, telling her father it was clear Charles did not love her.
Diana had suicidal thoughts and more than once drove to the famous chalk cliffs at Beachy Head on the south coast of England and considered "ending it all", her brother said.
Spencer, like his nephew Prince Harry, holds the media responsible for the car crash that killed Diana, saying the paparazzi were the "leading cause of her death".
The book was prominently displayed in windows and stacked on tables in London bookshops on Tuesday.
The Stg28 ($A53) hardback was offered at half-price in some shops and early sales were not brisk.
Broadcaster Piers Morgan threatened to sue over allegations of what he called "examples of brazen falsity" that he said he could prove were factually incorrect from when he worked for tabloid newspapers.
The royals have spent decades putting that period of crisis behind them.
The warmer, more personable approach of William and Harry - and, in recent years, the King - is a legacy of that time.
Some critics have questioned why Spencer felt the need to revisit the pain and trauma 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".
