More Alastair Campbell diaries, you say? And from so long ago? Yet they are endlessly fascinating even, I suspect, if you think the Blair government is ancient history.

Indeed, Campbell’s personal story is as engrossing as the political one. The most moving parts of these diaries are his father’s death and his own struggle against depression. The book covers the time from when he left Downing Street after the Iraq war until the 2005 election, when he came back to work full-time on the Labour campaign. In between, he was half in and half out – hence the title Outside, Inside – still advising Tony Blair but technically not working for him.

He is an unflinching and plain writer, and I mean plain as the highest praise. The reader never has to stop to re-read a sentence to try to work out its meaning. And he is an extraordinary diarist. The sheer volume of words is remarkable, coming from someone who was so often so absorbed in the “total war” of politics, and in so much other writing during the day.

In this volume this produces a peculiar effect, because one of the big tasks on which he was engaged was transcribing and editing his diaries from the time he started working for Blair in 1994, so we have the diary of a diarist working on his diaries.

He comments on how the same themes keep coming back from years before. Indeed, the reader notices that Neil Kinnock is always angry; everyone is always tired, or looks tired; immigration and asylum are always difficult, a premonition of the EU referendum more than a decade later (and the central European countries joined the EU only in 2004).

Above all, there is the endless groundhog problem of the relationship between Blair and Gordon Brown. It is on this subject that this book makes history. Campbell’s diaries are a unique document, as such a voluminous contemporary record from someone so close to the Prime Minister. Unlike Blair’s own memoir, or the more recent one of Ed Balls, one of the chief players on the other side, Campbell provides testimony from the day things happened – unedited by memory and hindsight.

This time, there is a distance between Campbell and Blair because they are not in each other’s company every day. Which means that, just as the Blair-Brown drama reaches its crisis, there is a muffled quality to the story. Campbell didn’t appear to know at the time about the dinner in November 2003 hosted by John Prescott, at which Blair promised to hand over to Brown the following year if the Chancellor were more supportive. Prescott didn’t tell Campbell about it until January 2004.

It wasn’t until April that Blair himself admitted to Campbell that: “the only way I was able to get him [Brown] to come into line last November – tuition fees, foundation hospitals etc – was to say that I wasn’t sure whether I was going to stand next time.” Campbell noted: “He did that thing he does, to indicate he may have been devious, when he appears to raise both eyebrows, the left one slightly more than the right, and smile.”

By this point, Blair needed Campbell to help him to tell Brown that he had decided to stay on and fight the 2005 election after all – on the grounds that Brown had failed to fulfil his side of the bargain, by “diddling” and obstructing Blair’s policies.

The rest we know: Brown was furious, saying to Blair in words that damagingly became public: “There is nothing that you could say to me now that I could ever believe.” Or, as Brown put it to Campbell: “You can only take so many lies from someone.” But then Campbell helped to persuade Brown to come out of his sulk to work with Blair one last time in the 2005 election campaign in return for the promise of keeping his job and for Blair endorsing him as his successor.

What we didn’t know, though, was how close Blair came – again – to sacking Brown in January 2005. At one point Prescott was signed up for the plan: “Sack GB, then force a [Labour leadership] contest partial or full. Then election.” But it dribbled away, and by the beginning of March Campbell wrote: “It was pretty clear that no matter how often we discussed it, TB was not going to put him out, and so it was vital we brought him into the centre of the campaign.”

Another new disclosure is of Brown’s attempts to lure Campbell away from Blair’s side. He praised Campbell inordinately, suggested he should become an MP, or a minister in the House of Lords. “I despair for progressive politics and so do you, I am sure of it,” Brown told Campbell at one meeting. At the next, at Brown’s home in Kirkcaldy, he said: “We still have the right wing setting the terms of the debate, and that was one of the things we used to talk about changing when you were at the Mirror and I was writing for the Record. We have not shifted the country leftward when we had the chance. We raised tax for the NHS but we then wasted the opportunity the debate gave us … I don’t expect you to reply but I think you are disappointed too, that we haven’t reshaped things as we hoped.”

Campbell didn’t reply but he didn’t agree – he felt things had changed: the Conservatives were now afraid of promising to cut public spending and were pretending to support the minimum wage. But he was tempted by opinion polls that suggested Labour would do better if Blair gave way to Brown before the election. He asked Blair if he should stand down. Blair was annoyed and said, “I would honestly go if I thought it would help. But I don’t, and I’m not sure you do.” Instead, Campbell persuaded Brown to sign up for a joint election campaign, a deal which carried Labour to its third successive victory.

In his introduction Campbell writes that TB-GB “is one of those political relationships historians, political students, writers and film-makers will look at for a long time to come”. Should Blair have sacked him? “Even having had a ringside seat, and with the benefit of hindsight, I still don’t have a settled view. Some days I think he should have done. Other days I don’t.”

But he rightly observes that in that relationship “we can perhaps see the seeds of Labour’s current difficulties under Corbyn”. Brown’s positioning of himself just to the left of Blair, constantly implying that Blair was untrue to Labour values, paved the way for his own failure, and then for the failures of his successors. But Blair let him do it.