Donald Trump has always alternated between snarling at reporters and fawning over them. During his time in the White House he defamed Maggie Haberman as a third-rate drudge or a “crooked Hillary flunkey” and tried to hack her phone to unearth the sources for her revelations about him in the New York Times. He once tweeted an unflattering photograph of her; whenever he saw her on CNN he sneered at her smudgy specs. His animosity amounted, in Haberman’s opinion, to a “fixation”. Yet although Trump knew she was writing a book about his ignorant, incompetent and often insane conduct as president, he welcomed her to his Florida country club and during their last interview remarked to his aides: “I love being with her, she’s like my psychiatrist.”

Haberman dismisses the ingratiation, then reflects that Trump “treats everyone like they are his psychiatrists”. He vents experimentally in the hope that others will be able “to decipher why he was doing what he was doing”; while in office he “drove days of news based only on his reaction to people reacting to him”. The crucial difference from a therapeutic session is that this tantrum-prone patient is not hoping for a cure. Instead, his aim is to mystify and, with luck, to madden the world. The White House during Trump’s occupancy was a crib for the mentally stunted monster Freud called “His majesty the child” and cabinet officials spent their days dangling shiny objects to distract him as he “popped off about various topics and chased fragments of conversation about things he’d heard on television”. Haberman’s book is chockablock with scoops, comprehensively leaked to the press before publication, but what singles it out from the competition is its perceptiveness about Trump’s character and the way his private vices became public menaces.

Officiating as a harassed shrink, Haberman diagnostically reviews Trump’s early life, when his manias and self-delusions were already blatantly evident. His mentor Roy Cohn, a corrupt and perverse attorney with a scar-pocked face and the habit of deliciously licking his lips as he uttered curses, taught him the uses of “emotional terrorism”. Negotiating with mafia thugs and political bosses during his years as a property developer in New York, Trump devised the tricks that have served him ever since: brazen lying, performative rage, chaotic plotting that sets allies at odds with one another. These tactics expose his gamesmanship, since power for him means unhindered play, kept going by double and triple bluffs. Hence his current claim, after the FBI raided his country club to retrieve the cartons of contraband nuclear secrets hidden there, that he could declassify such documents simply by thinking that he had done so.

Trump’s fabled, probably fabulated wealth also amounts to no more than random zeroes dancing in his head: his net worth, he admitted in 1991, “fluctuates with attitudes and feelings, even my own feelings”, since billions of dollars are merely “mental projections”. This confidence man defies the suckers to believe in him or vote for him and he mocks them when they are dim enough to do so. “Look at those losers,” he sneered at the customers who squandered their welfare cheques in his Atlantic City casino – though Trump himself, bankrupted by the ill-managed venture, was the ultimate loser. “They’re fucking crazy,” he often muttered as he basked in the baying adulation of the mobs at his election rallies.

Trump lives, Haberman argues, in “the eternal now”, which is why in his White House no one did any long-term planning. But he is also held captive by an “eternal past” of grudges and grievances, re-enacted in assaults on those who supposedly slighted him. Barack Obama, whom he envied and therefore despised, was exorcised in a hygienic rite: Trump replaced the Oval Office’s en suite toilet because he refused, Haberman suspects, to entrust his arse to the seat used by “his Black predecessor”. On other occasions he seems crassly sadistic or downright evil. He wanted his notional border wall painted black to absorb and reflect the sun’s heat so that the skin of migrants would burn and blister when they touched it. Hitler, he proclaimed during a presidential visit to Europe, “did a lot of good things”.

For all Trump’s belligerence, in Haberman’s view he remains fragile and fearful. He dispatched White House valets to fetch Big Macs for his dinner because fast-food restaurants wouldn’t know who they were serving and therefore were less likely to poison him. In the 1980s he required the models he dated to take Aids tests before he would condescend to have sex with them; he still rubs his hands red-raw with disinfectant wipes meant for use on non-porous surfaces (which may be the only symptom of guilt he has ever exhibited). At his pettiest, squeamish phobias such as these shade into prissy vanity. In France in 2018 Trump cancelled a visit to a cemetery for the American war dead when the weather changed: he explained that he didn’t want to get his hair wet in the rain. In his first television address about Covid-19, the prospect of an impending plague mattered less to him than “a visible spot on his white shirt” that he noticed just before the live broadcast began. He took the pandemic as a personal slight: his petulant refrain throughout those two terrible years was: “Can you believe this is happening to me?”

Trump “used the government”, Haberman concludes, “as an extension of himself”, treating it as a private enterprise that serviced his appetites, threatened his detractors and enriched his family firm. Even more destructively, he “ushered in a new era of behaviour” by turning hatred into “a civic good”; although he began with puerile name-calling, the vituperation has advanced beyond rhetoric and he no longer bothers to euphemise his appeals to militias such as the Proud Boys and the Oath Keepers. It is all at once alarming and absurd, elated by the imminence of a spectacular apocalypse. The Trump show, as Haberman remarks, has “a menacing psychological-thriller score and a sitcom laugh track” playing simultaneously. This is the way the world ends, with both a bang and a hollow, cynical guffaw.

• Confidence Man: The Making of Donald Trump and the Breaking of America by Maggie Haberman is published by Mudlark (£25). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply