Pamela Hicks nee Mountbatten was 17 when her father Lord Louis Mountbatten came to India as the last Viceroy.
She saw a "profound relationship" developing between Nehru and her mother Edwina Ashley.
"She found in Panditji the companionship and equality of spirit and intellect that she craved," Pamela says.
Pamela was keen to know more about the relationship.
But after reading Nehru's inner thoughts and feelings for her mother in his letters, Pamela "came to realise how deeply he and my mother loved and respected each other".
Pamela says she had been "curious as to whether or not their affair had been sexual in nature" but after having read the letters, she was utterly convinced it hadn't been.
"Quite apart from the fact that neither my mother nor Panditji had time to indulge in a physical affair, they were rarely alone. They were always surrounded by staff, police and other people," Pamela writes in "Daughter of Empire: Life as a Mountbatten".
The book, first published in the UK in 2012, has been brought out in India as a paperback by Hachette.
Lord Mountbatten's ADC Freddie Burnaby Atkins also told Pamela later that it would have been impossible for Nehru and Edwina to have been having an affair, such was the very public nature of their lives.
Pamela also writes that while leaving India, Edwina wanted to give Nehru her emerald ring.
"But she knew he would not accept it. Instead, she handed it to his daughter, Indira, telling her that if he were ever to find himself in financial difficulties - he was well known for giving away all his money - she should sell it for him," the book says.
At a farewell party for the Mountbattens, Nehru said while addressing Edwina directly, "Wherever you have gone, you have brought solace, you have brought hope and encouragement.
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