The time for a balanced and dispassionate assessment of the New Labour record is not yet upon us. It may be several years before contemporary historians are able to stand back with sufficient distance to do justice to the achievements and failings of the remarkable phenomenon that was set in train by Tony Blair’s election to the leadership on 21 July 1994. But when they do, the unexpurgated diaries of Alastair Campbell are likely to be the single most valuable source of detailed and informed observations from the heart of the project.

This is only the first volume and it is not for the faint hearted. Those of us who knew Alastair was keeping a diary had no idea just how comprehensive it was. How he found the time or the stamina to record so much detail at the end of long and exhausting days is a mystery, but the same can be said of all the best diarists of the modern political age. We should just be grateful that he did.

Few will read every word. Let me show some rare candour for a reviewer and admit that I haven’t, and indeed probably never will. But that is not the point. As a reference tool, as an academic resource and for the pure pleasure of it, Prelude to Power can be dipped into endlessly. Every page carries insights and observations that are both informative and entertaining. If for those who were closely involved at the time, either as participants or observers, it sometimes seems like a series of Groundhog days, well many of them were good days and are worth reliving. Others sound like days from hell and that’s clearly what they felt like at the time.

Before assessing the true value of any diary, however, the first question that has to be resolved is just how self-serving and, by implication, how compromised it is as a reliable source. All diarists believe they are taking part in something of significance, otherwise why bother? Some are ready to acknowledge that their own role in those events is as a bit player. The magnificent volumes from Chris Mullin fall into this category, and at a far more modest level I hope my own efforts (The Spin Doctor’s Diary, 2005) do the same. Alastair Campbell, on the other hand, was self-evidently a central figure. What makes Prelude to Power so valuable is that he never seeks to exaggerate his own role and is usually ready to admit when he’s made a mistake or failed to perform as well as he could have done.

Of course Alastair has an ego on him. He could not have done the job he did for so long if he didn’t. But when others told him as much – and Cherie Blair and his own partner, Fiona Millar, were often ready to remind him that he wasn’t infallible – he records it as faithfully as everything else. Politics is an ego-filled environment. I remember the look of incomprehension on Tony Blair’s face when I complained to him about the extraordinary self-obsession of one particular individual. What did I expect, he asked? That was in 1999. By then Blair had become hardened to the endless personality clashes and feuds, although at the start of his leadership they were a source of intense frustration and sometimes anger.

Like Blair, Campbell found the competing agendas of the senior politicians around him maddening and self-indulgent but he managed them with a remarkable degree of skill and patience. As a result Alastair was the one man that everybody still talked to no matter how bad things got and who was treated with respect and even affection by all who knew him well. Those who have only seen him through the prism of the media as a bully and a megalomaniac may find that hard to believe, but it happens to be true.

Prelude to Power provides a mass of detail that had to be cut from The Blair Years, published in 2007, for reasons of space. It also reveals what Alastair thought and wrote about Gordon Brown and his team but didn’t include in the earlier volume out of loyalty to the new party leader. Had Brown and those closest to him shown the same loyalty to Blair then the record of New Labour’s 13 years in power would have been very different. This volume, and no doubt those that are to follow, will make uncomfortable reading for all those who allowed Brown’s misplaced sense of resentment to influence their behaviour for so long.

The idea that all the tensions and rows were somehow ‘six of one and half a dozen of the other’ deserves to be debunked and a fair reading of the 750 pages published here will go someway towards that.

Too often it is the questionable behaviour of some of the big names that stands out, but there is space, too, for the many other people who gave their all to see Tony Blair elected. Today new leaders are, with luck, in a prelude to power of their own. They can learn much from this book about how to avoid the mistakes of the past, but just as importantly about the conviction, imagination and sheer hard work required to turn the ambition for power into the reality.