Standing 6ft 8in tall, James Comey – “the FBI giraffe” as he calls himself – has made a career out of seeing over the heads of lesser men, his eyes fixed on glimmering legal ideals of probity and propriety. In this book, before his ejection from the FBI, he occasionally looks out of his office window on Pennsylvania Avenue, bypasses the luxury hotel that Trump has opened blocks from the White House, and ponders the distant Washington monument, a pristine marble shaft that points at the sky. Comey, I suspect, feels an affinity with that aspirational obelisk.

His height makes him an awkward fit for the smaller-scaled world the rest of us occupy, and his memoir abounds in clumsy physical upsets. Wearing resoled shoes that boost him by half an inch, he miscalculates how far he needs to duck and bangs his head on the lintel of a door as he enters a solemn White House conclave, which leaves him trying to finesse a trickle of blood from his skull during a meeting with George W Bush. Embraced by the diminutive Loretta Lynch, Obama’s attorney general, he feels her head nudging his navel. Trump, grandly enthroned behind “a large wooden obstacle”, humbles Comey by assigning him to a child-sized chair, from which his bony knees protrude to prod the presidential desk.

Comey used his height defensively at a White House reception when Trump tried to lure him into complicity with a hug, an unwanted and inappropriate act of collusion between two branches of government that ought to remain separate. In a video of the incident, you can see Comey cautiously hold back and then, when Trump advances, swivel sideways; it’s a nimble manoeuvre, spoiled a little when Trump grabs him again, pulls him down and, like a mafia don ceremoniously initiating a loyalist, appears to give him a kiss. In their first meeting, Comey had to brief Trump about the unverified Steele dossier and its tale of Moscow hookers and a purported golden shower. He describes the occasion as “an out-of-body experience”, and out of his body – abstracted into what Judge Judy calls “a truth machine” – is probably where this cerebral man would prefer to be.

Comey’s character, with its stiff-backed rectitude, is a reflex of his stature. Sent to his room at the age of seven for some forgotten infraction, he apologised to his mother in a note, vowing: “I will be a great man some day.” Aware of his sanctimonious reputation, he humanises himself by relating traumatic events that shocked him into sympathy with the victims of crime. He was bullied at school, and at home he was once held hostage by an armed burglar; he and his wife lost a newborn son to an illness that might have been prevented. He values humour because it involves the admission of vulnerability. Trump, Comey says, is incapable of laughter and prefers to sneer, whereas Robert Mueller, the special counsel charged with uncovering presidential malfeasance, possesses “a grimace that passes for a smile”.

Despite these concessions to human weakness, Comey’s principles are as inflexible as his elongated limbs. He theorises about what it takes to be a leader, always indirectly aiming his remarks at Trump, who is likened to the Cosa Nostra bosses Comey jailed during his days as a New York prosecutor – an ignorant thug whose tantrums and rants make up for gnawing personal insecurities. Comey points out that Trump soliloquises unstoppably and incoherently, assuming that the silence of his browbeaten audience signifies assent; his constant lying seals him in an impermeable “cocoon of alternative reality”.

Although he doesn’t say so in the book, at college Comey wrote a thesis on the existential theologian Reinhold Niebuhr, who believed that Christians should “enter the political realm” to combat wrongdoing. On Twitter, where Comey has published landscape photographs that serenely confirm his belief in natural order, he even used Niebuhr’s name as his alias – a more high-minded pseudonym than John Barron, the identity Trump adopted in the 1980s when he phoned gossip columnists with updates on his sexual exploits. (Later, he leeringly recalled the imposture when he chose a name for his youngest son.)

Niebuhr, outfacing Hitler, understood both the necessity and peril of political engagement. Comey, resisting Trump’s efforts to suborn him, more naively believed that the law could be enforced without partisan bias. But how do you hold to account a man incapable of feeling either guilt or shame, who boasted in 2016 that he could shoot someone on Fifth Avenue without losing votes? Comey had to operate in a society where “divine deterrence” no longer has power; even the fear of a jail term can be dispelled by a pardon, as Trump signalled a week ago by exonerating Scooter Libby, who had been prosecuted by Comey for lying to investigators.

When working at the Department of Justice as deputy attorney general, Comey sometimes allegorically and alliteratively introduced himself at social gatherings as “Jim from Justice”. He admires the blindfolded, nobly impartial figure of Justice on top of the Old Bailey, yet his own experience demonstrates that someone with bandaged eyes is likely to stumble into disaster.

When Comey reopened the FBI scrutiny of Hillary Clinton’s emails shortly before the election, he assumed she would win, and wanted to ensure that her victory did not look illegitimate. At the same time, he inexplicably chose to keep quiet about the Russian meddlers who were helping Trump’s campaign. Clinton blamed Comey for sabotage; Trump first beamed about receiving an inadvertent leg-up, then fired Comey anyway – allegedly for declining to prosecute Clinton, actually for probing his own links with Russia. Clinton lost, Comey lost, but so did their country, which, with a government run like the mob, is now hurtling towards a constitutional crisis.

Asked at a Senate hearing whether his decisions might have been responsible for the mess, Comey confessed that the idea made him “nauseous”. His daughter later told him he should have said “nauseated”. I’m not sure she was right. If you’re nauseated you throw up, and America has not yet projectile-vomited Trump; feeling nauseous is a queasier and less emetic state that leaves you sick to your stomach and perhaps also sick at heart. Comey’s book ends by hoping that the current institutional “forest fire” will “spur growth” and “new life” when America regenerates itself, as it usually does. Maybe so. Meanwhile, I think he deserves to suffer those qualms in his gut for a little longer.

• A Higher Loyalty: Truth, Lies and Leadership by James Comey is published by Macmillan (£20). To order a copy for £17 go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99