Andrew Morton said Princess Diana never called her second son a "spare" but a "backup in the nicest possible way."
The Duke and Duchess of Sussex have received an invite but have yet to confirm their attendance at the ceremony.
Prince William is next in line to the British throne followed by his son Prince George with the Duke of Sussex fifth.
The Duke of Sussex was in London to attend the four-day preliminary hearing of his privacy case against Associated Newspapers Limited.
The Sussexes have only until after King Charles III's coronation on May 6 to stay at the cottage.
Associated's lawyer Adrian Beltrami told the High Court in London the claims, some dating back 30 years, were outside a time limit for legal action.
The Sussexes' children received their HRH titles when King Charles III became the new monarch in September 2022.
The Sussexes raised serious claims against the Firm in their Oprah Winfrey interview and in their "Harry & Meghan" docuseries.
The Duke of Sussex is suing Associated Newspapers Limited for breach of privacy and illegal information-gathering techniques.
Britain's Prince Harry has accused Buckingham Palace in court documents of withholding information from him about phone-hacking and said he was exposing the alleged wrongdoing by tabloid "criminals" out of love for his country.
Britain's Prince Harry arrived at London's High Court on Monday for a hearing against the publisher of the Daily Mail newspaper over alleged phone-tapping and other breaches of privacy.
The first hearing in a lawsuit brought by Prince Harry, singer Elton John and other high profile figures against the publisher of the Daily Mail newspaper over alleged phone-tapping and other breaches of privacy, is due to begin on Monday.
"You can't come into the country without meeting certain criteria and one of them relates to drug use," Samuel Dewey said.
"All I had ever wanted was privacy. In order to get it, it seemed that an exceptionally detailed 500,000-word book was in order," reads the synopsis.
Oil refinery workers and bin collectors in France have gone on a strike, with another day of protests planned for Thursday just before King Charles's visit.
The Duke of Sussex admitted to having used cocaine, marijuana, and magic mushrooms in the past.
The Duke of Sussex is in a legal battle with the government over a decision not to allow him to personally fund his police protection while in the country.
Donald Trump had reportedly pursued Diana and tried to woo her following her divorce in 1996 by sending her huge bouquets.
The Sussexes have yet to announce their decision if they will attend the ceremony or not.
Prince Harry will on Friday seek to win his libel claim against publisher Associated Newspapers without the case going to trial, his lawyer said, as the British royal steps up his legal battles against the tabloid press.
Buckingham Palace changed Archie and Lilibet's titles from Master and Miss to HRH Prince and HRH Princess.
A representative for the Sussexes confirmed they have been invited to the coronation on May 6.