A second book published Monday accuses India's premier of being weak and unable to stem corruption on his watch, further embarrassing the ruling Congress party in the midst of a bitter poll campaign.
The new book has been written by India's former coal secretary P.C. Parakh who retired in 2005 and was later implicated in the so-called Coalgate scandal related to allocation of mining rights.
Titled "Crusader or Conspirator? Coalgate and Other Truths", the book says an indecisive Prime Minister Manmohan Singh overlooked suggestions aimed at making the allocation process transparent.
"Unfortunately the prime minister was not able to control his ministers and my suggestion on coal block allocation was not accepted," Parakh, the coal ministry's former top official, said during the launch of his book in New Delhi.
"By continuing to head a government in which he had little political authority, his image has been seriously dented... although he has had a spotless record of personal integrity," he said.
The Coalgate scandal came to light in 2012 after the government auditor accused the coal ministry of underpricing coalfields and potentially causing losses to the treasury running into billions of dollars back in 2004-2006.
Singh, 81, who retires after this election, has denied any wrongdoing.
The new book comes just two days after a controversial book written by an ex-aide to Singh hit the shelves.
Sanjay Baru's book paints a damning portrait of a premier who was in office but "not in power" -- while Sonia Gandhi, party president and standard-bearer of the Nehru-Gandhi dynasty, called the shots.
The books come as India's multi-phase election is in full swing, with the opposition Hindu nationalist Bharatiya Janata Party (BJP) widely tipped to overthrow Congress after polling ends on May 12.
The BJP, led by firebrand conservative hawk Narendra Modi, Monday lashed out at the Congress leadership for keeping quiet despite the serious issues raised in the two books.
"In both the books, the PM is shown as somebody who was not in control. The (Gandhi) family can no longer run away from the responsibility of answering the serious questions raised in the books," BJP spokeswoman Nirmala Sitharaman told reporters.
Sonia Gandhi released a video late Monday evening in which she didn't comment on Singh but said: "Their (BJP's) vision is clouded with hatred and falsehood, their ideology, divisive and autocratic."
She asked the Indian voters to think from their "heart and decide".
"We are fighting this election for the heart and soul of India, to protect it from those who seek to change it and to divide us," Gandhi said.