* Book casts Indian premier as politically weak
* Unflattering account by insider embarrasses Singh
* Ruling Congress party already trailing in polls
By Tommy Wilkes
NEW DELHI, April 14 (Reuters) - A new book accuses India's prime minister of being weak and unable to stamp out corruption on his watch, the second recent attack by an insider that undermines the Congress party as it seeks re-election despite trailing in opinion polls.
The opposition Bharatiya Janata Party (BJP) has cast the world's largest ever election as a battle between its dynamic and assertive leader Narendra Modi and the relatively ineffective incumbent premier Manmohan Singh.
That impression was underlined in a book, published on Monday, called "Crusader or Conspirator? Coalgate and Other Truths" by P.C. Parakh, who retired as coal secretary in 2005.
It said Singh's inability to take on vested interests led to the so-called "Coalgate" scandal, which rocked his premiership.
It was the second book in the last week to portray 81-year-old Singh, prime minister since 2004, as a well-intentioned man of high personal integrity but one often unable to assert his authority.
The other, by a former media adviser to Singh, was greeted with glee by the BJP, and the party is expected to seize on the Coalgate book to try to land another blow at a crucial time.
The BJP is favourite to emerge as the largest party in an election that ends on May 12, although surveys conducted before voting began last week suggest it is unlikely to win an outright majority and will need to form a coalition with other parties.
Congress is headed for one of its weakest ever showings, the same polls showed.
"SOME RESIST, OTHERS SUCCUMB"
The Coalgate scandal erupted in 2012 after the public auditor questioned the government's awarding of mining concessions without competitive bidding, which it said unduly benefited chosen private and state companies and potentially cost the treasury billions of dollars in lost revenues.
Parakh said that Singh, though keen to introduce open bidding, could not tackle resistance from coal ministers in his administration. Parakh said he himself came under pressure from people interested in acquiring coal blocks.
"Pressures come in the form of enticements such as post-retirement assignments, partnership in business, bribery, blackmail or pure intimidation. Pressures also come from friends and relations," Parakh wrote in the book.
"Some can resist these pressures. Others succumb," he said, adding that at no time did the prime minister's office make recommendations or exert pressure in favour of any party.
The prime minister's media adviser declined to comment on specific allegations in Parakh's book, and referred to previous comments the prime minister made in parliament.
The media adviser said the government was helping the courts and police in their investigations into the affair, and that there was no case against anyone in the prime minister's office.
Singh has consistently denied his government did anything wrong, blaming the delay in introducing competitive bidding on resistance from coal-rich states ruled by opposition parties.
He has said the findings of the state auditor in the 2012 report were "clearly disputable".
Former media adviser to Singh, Sanjaya Baru, alleged in a book published on Friday that the prime minister allowed his authority to be undermined by Sonia Gandhi, president of the Congress party and standard-bearer of the Nehru-Gandhi dynasty.
In an interview on Monday, he commented on the prime minister's handling of various scandals under his rule.
"In all these cases he was aware of what was happening, he tried his best given the political limitations," Baru told TV channel CNN-IBN.
"Given those political limitations, he took the measures he was able to take and what he could not do, he could not do. So it was not as if he was blind." (Reporting by Tommy Wilkes; additional reporting by Aditya Kalra; Editing by Frank Jack Daniel and Mike Collett-White)