The Queen Mother died in her sleep in 2002 aged 101
The Queen Mother thought Edward VIII was "bemused with love" and "couldn't be reasoned with" at the time of his abdication, a biography says.
The book throws light on the 1936 abdication crisis, when Edward gave up the throne in order to marry American divorcee Wallis Simpson.
The official biography quotes from a previously unseen 1990s interview.
"You couldn't reason with him, nobody could. The whole government tried, everybody tried," she said.
"The only good thing is, I think he was quite happy with her."
Speaking to then Eton headmaster Eric Anderson, she said: "It was a terrible surprise to everybody when he decided that he had to leave.
"It was the whole Commonwealth who said no no, we don't want you to marry this lady.
"And it was just a terrible tragedy, it really was."
She added: "I wonder. I don't think he ever wanted to be king. I don't think he thought of it as something he ought to do. Very odd."
The 1,000-page biography, by William Shawcross, also takes extracts from a letter the Duke of York - later George VI - wrote to his mother, Queen Mary, revealing his fears.
Edward married Wallis Simpson in exile in France in 1937
"It has been awful for all of us, but much more so for you, when David (the king) has been trained for the great position he holds and now wants to chuck away," he said.
"I am feeling very overwrought as to what may befall me, but with your help I know I shall be able to carry on..."
Edward VIII had proposed a solution to the problem - a morganatic marriage where his wife would not become queen but take a lesser title.
The Queen Mother did not regard this as the answer.
In a letter to her sister May, she wrote: "I feel so sad, and yet there is only a very straightforward case - if Mrs Simpson is not fit to be Queen, she is not fit to be the King's morganatic wife."
The book also reveals that the Queen Mother hated the official title - Queen Elizabeth, the Queen Mother - that she adopted following the death of her husband George VI in 1952.
In correspondence with her daughter Queen Elizabeth, she described it as a "horrible name".
The book is to be published Friday.