In The Unfinished Revolution, Philip Gould, a key strategist and chief pollster during Labour's 1997 election campaign, reveals the extent of isolation the former Social Security Secretary experienced among her senior colleagues. She had, says Mr Gould, little support except from Peter Mandelson, himself and a few others "who felt that if she went, New Labour lost".
Ms Harman came under enormous pressure to resign over the affair. "Many, probably a majority of even Blair's office, thought she was wrong," he writes.
Mr Gould was summoned to see Tony Blair in January 1996 and asked whether Ms Harman should resign: "I said, 'She has to stay. We cannot allow old Labour to win'."
The book also sheds new light on Labour's abrupt change of direction on education, signalled just two days after Mr Blair's election as leader in July 1994, when he overturned the party's hard line against selection and pledged that inadequate teachers would be sacked.
According to Mr Gould, the change had been discussed with advisers beforehand. Mr Blair not only wanted to make clear that education would be a central priority but that he intended to "refocus Labour's education policy away from vested interests and towards standards".
Mr Gould, a political consultant and former advertising executive, is a central figure in the creation of New Labour and a key moderniser.
Despite his support for Harriet Harman in her hour of need, Mr Gould does not favour school selection.
Having himself failed the 11-plus and left school with a solitary O-level (eventually reaching university via further education college), he blames the secondary modern system for killing the potential of generations ofchildren.
"That is why I abhor selection at the age of 11, but also why I abhor the potential that is still being wasted by a comprehensive system that was so badly flawed in design and execution," he says.
"The Unfinished Revolution: How the modernisers saved the Labourparty, " by Philip Gould, is published by Little, Brown at Pounds 16.99.