The book, giving ammunition to opposition critics during the marathon vote that winds up May 12, paints a damning portrait of a premier who was in office -- but "not in power" -- while ruling Congress party president Sonia Gandhi called the shots.
Entitled "The Accidental Prime Minister -- The Making and Unmaking of Manmohan Singh," the book is by academic and journalist Sanjaya Baru, a former media advisor to the current prime minister, who left his post in 2008.
Traditionally, the leader of India's ruling party is also premier. But Gandhi, who led the campaigns that swept Congress to power in 2004 and again in 2009, turned down the job, fearing her Italian birth would become an explosive political issue as Hindu nationalists said her foreign origin made her unfit to rule India.
She handpicked Singh, known as a family loyalist, for the post but Baru said Gandhi's much hailed "renunciation of power" was more a "political tactic than a response to a higher calling".
Baru said Singh decided early on to "surrender" to Gandhi and quotes the premier as saying he had "to accept the party president (Gandhi) is the centre of power".
- Blind eye to corruption -
Critics have long charged Gandhi held the reins of power in the Singh administration but Baru's book is the first by a close advisor to the prime minister to make that claim.
In a strong criticism of the soft-spoken premier, Baru said Singh "averted his eyes from corruption" to ensure his scandal-tainted government's "longevity".
Singh, 81, who retires after this election, maintained the "highest standards of (personal) probity in public life", but turned a blind eye to the misdeeds of his ministers".
Congress has been rocked by a string of scandals involving misallocation of telecom licences to mining rights that have cost the public treasury billions of dollars, according to government officials.
Singh thought he could choose cabinet ministers but "he was defanged" as "Sonia nipped that hope in the bud", said Baru.
The author added that Singh had little authority over his cabinet and a senior bureaucrat would seek Gandhi's "instructions on the important files to be cleared by the PM".
Singh's spokesman, Pankaj Pachauri, told AFP the book "smacks of fiction" and alleged Baru misused a "privileged position" for "commercial gain". Baru replied he was "amused" by the reaction.
The book's release comes as India's multi-phase election is in full swing, with the opposition Hindu nationalist Bharatiya Janata Party (BJP) tipped to win. Results are due May 16.
Baru portrays Singh as an eminent economist who initiated India's first round of landmark economic reforms in the 1990s.
But Baru said he made one "fatal error of judgement" in "promising loyalty to (an undemocratic) hereditary succession".
In rare past interviews, Gandhi, widow of slain former premier Rajiv Gandhi, has made it clear she sees herself as torchbearer of the Nehru-Gandhi dynasty which has given India three prime minister.
She pushed her son Rahul Gandhi, 43, on to the political stage to lead the Congress campaign. But Rahul's reluctance to take up his political mantle and his wooden performances are seen as key factors in the BJP's expected victory.
The BJP, led by firebrand conservative hawk Narendra Modi, pounced gleefully on the book.
"I've been saying from day one the prime minister presides and madam (Sonia) decides. This has been proved by this book," BJP leader Venkaiah Naidu said.