The earl has made a series of claims about the King in Swan Song: Diana, My Sister - including that Charles said "we'll forget her soon enough" after she died - which has prompted a war of words between him and Buckingham Palace.
A palace spokesman said last week the King is "mindful that the pain of fraternal grief can cloud reason, affect judgment and colour memory in ways others do not recognise".
Asked about telling Harry and Prince William about the book, the earl told People magazine: "I can say that Harry is completely fine about it".
"He said, I hope there's nothing embarrassing about him in it. I said, 'There isn't'.
"It's natural that people so in the public eye are concerned how they will come across in the book.
"But I could reassure both of them that they're of course in the book, but really as examples of two little boys who had the most exceptional mother, I'm not in any way compromising their privacy at all."
It comes amid reports that Harry is planning a documentary about Diana for the 30th anniversary of her death next year.
The earl admitted to "concern" about how William and Harry would feel about his portrayal of their father, but added he is "not here" to write a "scandalous book" about the King.
He told the magazine he did "not really" have any fear about writing so candidly about Charles.
"I'm not interested in who people are in terms of status," he added.
"I'm looking at them as people. And the central person, the only one that really matters, is Diana."
The earl believes his sister would have liked the book.
"She always loved it when I paid compliments, and this is 350 pages of compliments," he said.
The book is published on Tuesday.
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