Alan Rusbridger has a claim to have been the most successful editor of The Guardian since C P Scott, who edited the paper from 1872 to 1929 and is still in a way its presiding spirit. During his editorship (1995–2015), Rusbridger steered the paper, often showing real courage, through a series of stormy stories and, for the most part, emerged with an enhanced reputation. He rode the wave of digitalisation at a time when ‘nobody knew anything’ about the future and won a bunch of awards for The Guardian’s website. He gave substance to the paper’s slogan ‘the world’s leading liberal voice’. He refused to introduce a paywall. When the New York Times went behind a paywall in 2011 and lost traffic, he believed the slogan had become reality.

Scott had prioritised reporting, famously saying that ‘comment is free, but facts are sacred’. Rusbridger did too, and his book begins with a worry – which comes to nothing much – about ‘fake news’. Scott believed that a newspaper ‘should have a soul of its own’. Rusbridger believes that too, writing that The Guardian had ‘a definite sense of what it believed in’.

Up to a point. During his editorship its commentary stretched from centrist to hard left. The latter pole is now less dourly guarded following the departure of Seumas Milne to become a close adviser to Jeremy Corbyn, but it still finds a voice in Owen Jones, who mixes commentary with hyperactive cheerleading for the Labour leader.

The mix of views found in The Guardian, from Jones through Polly Toynbee to Jonathan Freedland and Martin Kettle, is more than eclectic: The Guardian’s commentariat often offer different visions of society. They did not, as Rusbridger suggests, live in amicable acceptance of difference during his time in charge. At a meeting of the paper’s journalists to which I was privy, a bitter argument sprang up between David Leigh, the chief investigative reporter, and Michael White, the political editor (both now retired), over Leigh’s view that the House of Lords as a whole was corrupt. Rusbridger, chairing, let it play out.

Rusbridger’s calm, reserved manner allowed him to preside stoically over a paper at times convulsed with ideological dissension. It proved of infinite value when he was faced, early in his tenure, with a libel action brought by the Conservative government minister Jonathan Aitken. The action seemed destined for success and huge damages when, at the last moment, proof was found that Aitken had lied under oath. He was ruined, but found solace in Christianity.

Of greater lasting importance was the work, beginning in 2009, of the reporter Nick Davies, who discovered that journalists working at the News of the World were routinely hacking the mobile phones of celebrities, politicians and others. Davies’s researches proved accurate: in fact, he underestimated the extent of the practice, which turned out to be routine in other tabloid newsrooms too. The great service he and The Guardian did to journalism produced a torrent of spite: the ‘dog does not eat dog’ rule had been flouted, and thus open season on The Guardian was declared, with the Daily Mail in the lead.

The resulting furore produced an inquiry chaired by Lord Justice Leveson, from which a fascinating report issued – and a recommendation that a government-appointed regulator take over from the discredited, voluntary Press Complaints Commission. That has not succeeded: no major paper, including The Guardian, signed up to the state-approved regulator, though the affair made the tabloids more careful. In any case, all newspapers are now losing influence, together with circulation, and the internet has given rise to a new ecology of journalism.

By some way the most important act of Rusbridger’s editorship was his decision to publish a series of leaked documents, a decision that ushered in a new form of revelatory journalism – and new dilemmas for editors. It began in 2010 with the publication of documents obtained by WikiLeaks relating to the Afghan and Iraq Wars, including thousands of US military and diplomatic cables hacked by Private Bradley (now Chelsea) Manning. Their publication shed light on both the way the wars were conducted and their brutalities (though always those committed by the Americans), and the quotidian to and fro of US diplomacy. Ironically, since this was hardly the intent of the founder of WikiLeaks, Julian Assange, the publication of the documents showed that a significant part of diplomats’ work involves trying to improve respect for civil and human rights in authoritarian states.

The crowning event of Rusbridger’s tenure was the paper’s publication – with others, including the New York Times and Der Spiegel – in 2013 of some of the huge cache of files stolen by Edward Snowden from the US National Security Agency (NSA), for which he had worked as a contractor. Rusbridger claims that the revelation that both the NSA and the UK equivalent, Government Communications Headquarters (GCHQ), were able to monitor all citizens’ communications and choose to inspect those deemed suspect represented a triumph for democracy.

So it was, and legislation setting this practice on a surer legal footing and introducing greater safeguards on its use was passed in the USA and the UK without much public disquiet: the public, fearful of jihadist atrocities, thought they needed, and trusted, the secret services. But Rusbridger, emphasising what he sees as the absurdity of the state response, does not discuss the downside of the NSA leak: the fact that some of the hundreds of thousands of files downloaded by Snowden will have been of use to powers hostile to the Western democracies, such as Russia, and that both the NSA and GCHQ have had to re-engineer, at great expense, their systems in order to repair the damage and upgrade security.

Rusbridger also says that in conversations with him senior security officers tacitly admitted that they had overdone the horror of the leak and actually believed that national security had suffered little. In conversations I’ve had, probably with some of the same officials, I’ve heard a different narrative: there is a continuing concern that real damage has been done and could still be done. Rusbridger’s failure to admit that security is precious for democracies, that it has likely been compromised, and that Snowden could – and should – have revealed the extent of government monitoring without stealing vast numbers of files he did not read, is a significant omission.

Yet Rusbridger’s editorship was, in the main, a boon, characterised by much good reporting, especially on politics, robust commentary, the development of a fine website (owing much to the early work of the ‘visionary’ Emily Bell) and a keen eye for injustices. The Guardian remained high-minded (and highly educated: all but one editor from Scott onward have been educated at Oxford or Cambridge, usually after a good public school). High-mindedness attracts resentment but can be put to good use.

Rusbridger’s leaving, which he does not describe, was unhappy. His determination to have no paywall, investments in large workforces in the USA and Australia, the move to an office complex near King’s Cross, a shift to the Berliner format requiring special presses and the maintenance of a still-significant corps of high-quality foreign correspondents meant he bequeathed to his successor, Katharine Viner, a paper rapidly running through its financial buffer. The expected online advertising revenue disappeared into the maws of Google and Facebook. She has had to make cuts; the ensuing resentment meant that he did not after stepping down become, as expected, chairman of the Scott Trust, The Guardian’s owner. He did, however, become principal of Lady Margaret Hall in Oxford and chairman of the Reuters Institute’s steering committee. In Oxford, he has moved to make his college more accessible to students from state schools and ethnic minorities: a liberal reformer, still, much in the mould of C P Scott.