Spare a thought for the factcheckers at the publisher HarperCollins. For a regular political memoir, those guardians of textual accuracy might, you imagine, raise a query perhaps half a dozen times in each chapter. In the 750-plus pages of Boris Johnson’s Unleashed, however, I’d calculate they were probably working at a rate of nearly one red-inked alarm bell per sentence. No phrase that Johnson writes in this book comes entirely unlaced with hyperbole or self-serving spin. You’d hope the copy editors were being paid by the marginal note and had access to physiotherapy for RSI.

Take just the headline acts of the book (for which Johnson may expect to earn £3m) – those stories splashed earlier this week across the Daily Mail. The claim, for example, that the former prime minister had to be restrained from having British special forces invade the Netherlands in order to secure a batch of Covid vaccines impounded by the EU; or that after a game of tennis with David Cameron at Chequers the then prime minister reportedly vowed to “fuck him up for ever” if he supported the Leave campaign; or that Emmanuel Macron told him that Britain deserved a “punishment beating” for Brexit; or that the queen’s last words to him, two days before she died, were an expression of approval for his insouciance over the manner of his defenestration.

All of these claims, like almost everything else in this book, come with not even the hint of a documented footnote. You quickly realise that each one requires what was always the most preposterous of Johnson’s demands: that you should take him at his word.

There are many illustrative instances where the truth of contentious boasts has been established a thousand times. Exhibit A may be Johnson’s perpetual claim that without Brexit there would have been no accelerated vaccine rollout during the pandemic. No matter how many times it has been shown that the Pfizer and AstraZeneca vaccines would never have required EU approval (because of regulation that had been in place since 2012), Johnson never tires of repeating the fiction. He adds a clincher here. “It wasn’t long,” he claims, “before graffiti appeared on the wall in Portobello Road, west London. ‘Brexit saves lives,’ it said.”

It is exactly a decade since Johnson’s book The Churchill Factor, in which he tried to align his then nascent political ambitions with those of the great wartime leader. That book argued that Churchill – and by wildly hopeful extension, Johnson – offered a “resounding human rebuttal to all Marxist historians who think history is the story of vast and impersonal economic forces… one man can make all the difference”.

The intervening years, as this volume shows, have proved Johnson both right and wrong in that contention, to all of our costs. He no doubt made a difference; public life has undeniably become more fractured, venal and shallower for his leading role in it. But his time in office also proved one of the most trenchant of Marx’s observations that “history repeats itself first as tragedy, second as farce”. Once again here Johnson reveals himself, in his desperate pastiche of his hero’s gambler’s instinct, to be incapable of any register other than snark – you won’t find any convincing notes of solemnity or compassion in this book, because where is the Bunterish snigger in that?

The overarching irony is the fact that the fates presented Johnson with all the circumstances in which he might have proved his mettle as a leader: the great struggle to normalise relations with the EU and the existential crisis of the pandemic. In his reflections on both he shows himself hopelessly wanting. Looking back at his infantile resolution to “Get Brexit done”, he notes that his efforts to ingratiate himself with President Macron involved not negotiating the small print of an effective trade deal or cross-border security, but in suggesting a hydrogen-powered Concorde and a bridge across the Channel.

Covid, meanwhile, saw him dwelling not on the strained infrastructure of public health but, inevitably, on his crammer’s knowledge of the classics: “‘Pericles died of the plague,’ I had earlier reminded my old friend Michael Gove,” he recalls, “and his spectacles seemed to glitter at the thought, like the penguin in Wallace & Gromit.” His own life-threatening spell in ICU caused him briefly to “wonder blackly… [if] it was some kind of cosmic judgment on me, on my whole worldview”. That doubt doesn’t survive his return to health. Rather than, say, attend Cobra meetings, his primary sentiment remains that “I needed to bee-oing-oing back on to my feet like an India rubber ball… fixing the care homes, driving the quest for a cure…”

Johnson is not a man made for regret, of course. He repeats a wild belief that he would have won the 2024 election had he not been removed from office by colleagues suffering a collective “attack of the vapours”. And he bangs on, inevitably, about his “big idea” – levelling up – formed partly at Eton when he was confronted with the startling fact that hereditary toffs weren’t necessarily the sharpest knives in the drawer.

Countering this vaunted commitment to social mobility, of course, are the many occasions in which he proved himself the most enthusiastic promoter of cronyism. This, for example, is how he describes his appointment of Kate Bingham, “whom I had known since I was 18”, to head up the vaccine taskforce. “She was one of a bevy of brilliant and energetic Paulinas (alumnae of St Paul’s Girls’ School) who went around Oxford in the mid-1980s, terrorising and breaking the hearts of their male counterparts. She had married an old school friend of mine, Jesse Norman, and she had also been at school with my sister, the ubiquitous Rachel… it was precisely because I knew her that I knew she was superabundantly qualified for the job.” There is less to be said for Dido Harding, also a friend and the wife of a Tory MP, who was entrusted with spaffing £37bn on the entirely ineffective test and trace “moonshot” (in large part, it seems, because Johnson saw the potential for schoolboy jokes in the classical allusion of her Christian name).

Just occasionally he stumbles on the truth. At one point he mentions a piece written 20 years ago, which foretold that “the Tories will never recover until Boris Johnson leaves the stage”. You only have to glance at the current quartet of Conservative leadership candidates to see that his legacy – the hollowing out of principle and character in the party he led – is very much alive. And in this sense, you read this book with the growing nausea that its author still, despite everything, inevitably envisages a heroic sequel.

• Unleashed by Boris Johnson is published by William Collins (£30). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply