Blair Unbound
by Anthony Seldon with Peter Snowdon and Daniel Collings
669pp, Simon & Schuster, £25

The word doesn't occur in my dictionary, but at the heart of Anthony Seldon's dauntingly detailed second instalment of his history of the premiership of Tony Blair is the study of a cliquocracy: the entrusting of a government's fundamental direction not to the party he leads, nor on the whole to his cabinet, but to a group of dedicated and driven loyalists picked out by a prime minister in the service of a personal doctrine. Nothing entirely new about that: it's the method Margaret Thatcher lived by, and in the end, because of her isolation, died by. Churchill, too, had his much distrusted Lindemanns and Brackens, people with no great official status but with whom he felt happier and safer than most of those served up to him by the system. Without this kind of stimulus there must always be a tendency for political leaders to sink back into a kind of consensual inertia, where nothing gets done. But has any cliquocracy ever gone further than Blair's? After 583 pages of Seldon's text, I doubt it.

This, too, is the context for a message which Seldon, I sense, thinks more important than the one that the newspapers have so far extracted from this huge undertaking - which is what we knew already: that Blair plus Brown was a wretchedly dysfunctional marriage. Conventional accounts see Blair as a man who started brilliantly, soaring swiftly to a level of popular trust that none of his predecessors could match, and then, under the strain of events and unresolved problems, chiefly Iraq, began to founder and fade. Seldon sees it the other way round. It was only towards the end that Blair was unbound, that he knew at last what he wanted, strove for it, and would have found his way even further towards it had it not been for Brown. And this was largely because the clique that Blair had gathered around him in his second and third terms - the first was dealt with in a previous book - was simply better, more perceptive and more astute than the one he began with. Alastair Campbell had gone - and not before time; he'd become counterproductive - and Peter Mandelson, too, an even greater blessing in Seldon's analysis: indeed, Blair's dependence on Mandelson comes out in this book as almost as chronic and inexplicably misconceived as Cherie's on Carole Caplin. Other advisers had in these subsequent terms come to his side, including Jonathan Powell and Sally Morgan, successor to the much-missed Anji Hunter, and Campbell's less visible successor, David Hill - but above all Andrew Adonis, who becomes, as events develop, the hero of Seldon's book. He moved on to become a senior minister at Education, more influential because closer to Blair than the cabinet ministers ranked above him - but before that, in the second term, he was constantly at Blair's elbow as No 10 head of policy: "Adonis's relationship to Blair, which blossomed in the second term, was key to establishing the new agenda," Seldon says. "We were both Christian democrats as much as social democrats," Adonis explains in one of the most telling passages in the book, "with a keen sense of the futility of the old left-right British party system to reflect modern progressive society and politics, and I was always willing him on to be himself".

Next door, however, another clique was at work: the close allies and courtiers of Gordon Brown, of whom Ed Balls emerges as the noisiest, most conspicuous, and most hated by the coterie at No 10. The inescapable parallel here is the tension familiar from history between the court of an ageing monarch and that of a Prince of Wales desperate to get his hands on power before it's too late. Previous books on Blair have produced their own revelations - Robert Peston's especially, which had Brown telling Blair he could never trust him again - and this adds fresh ones, with Brown telling Blair, more than once, to F off and hand over the job. More than once, too, the No 10 crew incited Blair to sack his insubordinate subordinate, while those around Brown railed at him for failing to finish off Blair when he had the chance. And why did the principals shrink from taking this powerful advice? Oddly enough, Seldon suggests, because these one-time allies were still, deep down, fond of each other. An alternative view might be that they knew all too well the ill consequences for themselves if they succumbed to such guidance.

Here, as throughout the book, a quite extraordinary bank of evidence has been marshalled. It is based on hundreds of interviews at home and abroad, sometimes leading to far too much detail: it's a pity that Seldon couldn't abandon the silly practice familiar in Sunday newspapers where reporters tell you what participants at important events had for dinner and the fluids they washed it down with, as if this proved that they really knew all that went on - which of course it doesn't. There are more than 3,800 footnotes as demonstrations of the diligence of Seldon and of his two henchmen, who did much of the interviewing. But as ever on these occasions, the moments of revelation which have you rushing to the back of the book asking: "who on earth told them that?" often end in frustration. Almost half these footnotes (47%) are based on private interviews, private papers, private letters. If you struck out all the testimony to which witnesses declined to attach their names, the book would be very much thinner. In the end it boils down to whether or not you trust Seldon's judgment on what among this material is truly believable.

I think that for almost all of the time, I do. My problem with Blair Unbound is a different one: that it asks one to accept that people such as Adonis, or Alan Milburn, or John Birt (who, fresh from the BBC, comes to think great radical thoughts on Tony's behalf) - men who constantly press for more and more radical change - are necessarily right. Sometimes they are; but the system, of proposition by cliques, with those who protest or reject billed as hopeless reactionaries, is the one that gave us the poll tax, the PFI and other gleaming notions that didn't work out as planned. On top of that there's the weight and stress that constant change imposes on people in Health and in Education, who have hardly implemented one great reform before they're engulfed in another. Seldon's book is dedicated to his father, Arthur Seldon, who with Ralph Harris ran the fundamentalist free market Institute of Economic Affairs: it quotes the Economist saying in its obituary of Arthur Seldon, who died in 2005, that he was "the architect of Thatcherism and Blairism". One can see how this might sway his son in favour of those who propel a prime minister to market-oriented solutions, but there's no obvious reason why the rest of us should swallow it whole.

Since the book was published, reports have appeared that Brown's government is retracting earlier proposals for extending city academies and for greater private involvement in the NHS, causing Blairite true believers such as Milburn and Stephen Byers to mutter about treachery. Seldon, I guess, would take the same view, and would see it as unhealthy backsliding. Yet nothing in his huge and studious history comes anywhere close to proving that it's really all timid retreat, rather than, as I suspect, simply a belated outbreak of common sense.

· David McKie's Jabez: The Rise and Fall of a Victorian Rogue is published by Atlantic.