Asked by reporters in Amethi about Sanjaya Baru's book "The Accidental Prime Minister: The making and unmaking of Manmohan Singh", Priyanka said: "Only Manmohan Singh was super PM."
The book by the prime minister's former media advisor says Manmohan Singh was in office but "not in power", and key decisions were taken by Congress president Sonia Gandhi.
Earlier, Manmohan Singh's daughter Upinder Singh described the book as "nothing but a stab in the back".
In an interview to a newspaper, she accused Baru of violating trust and branded the book an "unethical, mischievous" exercise.
"While I am not speaking on behalf of the prime minister, we are extremely angry at the exaggerated level of access the author has ascribed to himself," she was quoted as saying.
Bharatiya Janata Party's prime ministerial candidate Narendra Modi, however, used Upinder Singh's statement as an affirmation of the allegations in the book.
"I read in the papers that the prime minister's family is unhappy with Sanjay Baru's book. His daughter said Baru betrayed, which means the facts are right," Modi said at a rally Tuesday.