Liz Truss had a question for Anthony Seldon when he bumped into her at last year’s Spectator summer party. “Why are you writing a book about me?” she demanded to know. I’m sure many of us would share her wonderment. For almost 200 years the dubious distinction of being our shortest-serving prime minister belonged to George Canning, whose 119-day tenure was ended by the grim reaper. The only thing that died at the end of Truss’s 49 days (starting on 6 September 2022) was the Tories’ reputation for economic competence, although it’s fair to say it had been on life support for a while. The chaotic Conservative leadership changes in the summer of 2022 had produced new records for ministerial impermanence; an education secretary lasting for 36 hours; Grant Shapps as home secretary for six days. Truss may not have outlasted the Daily Star’s famous wilting lettuce but, just as she qualifies for a lifetime of close protection and annual gatherings at the Cenotaph, like all contemporary prime ministers, she gets an Anthony Seldon book on her time in office.

A short one, you would have thought. The scaffolding for this flimsy construction consists of 10 commandments that Seldon drew up in 2021 to analyse the perils of a leadership role he’s been writing about for 40 years. The Truss premiership is judged under chapter headings such as Command the Big Events, Maintain a Reputation for Economic Competence and Avoid U-turns. As you can imagine, she struggles a bit on all of them, hence the subtitle How Not to Be Prime Minister.

This is a brisk and readable account apart from a torrid first 50 pages covering the leadership contest that got Truss to No 10. It’s not so much the candidates as the courtiers who raise the hackles. A cavalcade of oddballs – Gavin Williamson, Mark Francois, Dominic Raab, Nadine Dorries, Suella Braverman, Jacob Rees-Mogg, David Frost – flit across the page, screaming arrogance and self-importance. It’s a cast list too gruesome to be entertaining.

Liz Truss won principally because her tiny electorate (97% white, 50% over 60, mostly resident in the south of England) still worshipped Boris Johnson and she’d been careful not to join the regicide. She did have a plan. With inflation at a 40-year-high and borrowing becoming more expensive by the day, she’d introduce £45bn of unfunded tax cuts to principally benefit the already wealthy. While the folly of this was apparent to most sentient beings, including the man who’d been her main leadership rival, Rishi Sunak, her critics were dismissed as the “anti-growth alliance” or purveyors of “abacus economics”.

Seldon reveals where her demented notions came from, castigating the Institute of Economic Affairs, in particular, for continuing “to move away from the academic scholarship that characterised its early years to appeal to a certain breed of rightwing politician promoting specific causes such as Brexit, climate change scepticism and opposition to the nanny state”. Seldon should know; his father co-founded the IEA in 1955. Today it’s registered as an educational charity with a duty to engage equally with all political parties. While it “engages” frequently with Tory donors and the tobacco industry, it has yet to make a single utterance in support of Labour.

Its director general in 2022 was a man called Mark Littlewood, a friend of Truss since their student days when both were Liberal Democrats. It was he and his chums who inspired the plan that was to become the disastrous mini-budget, delivered by her closest political ally, Kwasi Kwarteng. Unlike Truss, Littlewood and the Daily Mail (“AT LAST! A TRUE TORY BUDGET”), the former chancellor emerges from these pages with dignity. He was certainly too deferential for too long and made a catastrophic error by telling the BBC’s Laura Kuenssberg that there was “more to come” after the unfunded tax cuts sparked the inevitable financial meltdown. But Truss had cleared his comment and was insistent there should be no balancing spending reductions (‘No! No! No!”). All of which is just as well given that George Osborne only managed £30bn of cuts, whereas austerity 2.0 would have required £45bn initially, rising to £75bn by the time the retreat was sounded. Yet it was Kwarteng who went quietly, having been thrown under the bus by his boss in a bid to save her skin. And it was Kwarteng who’d been forced to sack Tom Scholar on her orders. Truss had developed a grudge against the Treasury’s well-liked and highly respected permanent secretary when she’d worked at the department under former chancellor Philip Hammond. As Seldon explains, Scholar was considered indispensable by many in Whitehall and the prime minister was soon to find out why.

One of the more esoteric components of the shitstorm created by the mini-budget concerned the threat to something called liability-driven investments (LDIs) – basically a mechanism through which pension funds secured income-generating assets. When it looked as if pensions were about to go the same way as mortgages, Truss admitted that she’d never heard of LDIs and bewailed the lack of warning from Treasury officials. There was one expert who could have provided chapter and verse on the subject – Tom Scholar.

Seldon’s conclusion on Truss isn’t totally damning. He rates her more highly than Johnson, though that’s a pretty low bar. She is at least credited with being politically savvy, but “her savviness was micro and self-centred when it needed to be macro and inclusive”.

Readers may remember another Truss book being the subject of one of Keir Starmer’s more memorable remarks before it was published. “A book is being written about the prime minister’s time in office,” he said during prime minister’s questions. “Apparently, it is going to be out by Christmas. Is that the release date or the title?”

Truss never made it to Christmas, going even before the first John Lewis ad. The voters of South West Norfolk finally ended her parliamentary career in July. Those who read this book may well think they spoke for Britain.

• Alan Johnson is a former home secretary and Labour MP. His new book, Harold Wilson, from The Prime Ministers series, is out on 26 September (Swift Press, £16.99). To support the Guardian and Observer, order your copy at guardianbookshop.com. Delivery charges may apply.

• Truss at 10: How Not to Be Prime Minister by Anthony Seldon, with Jonathan Meakin, is published by Atlantic (£22). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply

• This article was amended on 9 September 2024. An earlier version referenced a remark by Keir Starmer at PMQs saying it was about Anthony Seldon’s book. In fact the remark related to another planned book about Liz Truss.