The essence of irony, Henry Fowler wrote in “A Dictionary of Modern English Usage,” is that it “postulates a double audience” — one that’s in on the joke, and another that isn’t. The title of Philip Rucker and Carol Leonnig’s new book, “A Very Stable Genius,” is thus savvy marketing. It’s possible to imagine both Donald Trump’s detractors and his admirers eagerly grasping a copy.

The admirers will not make it past the table of contents. Among the chapter titles: “Unhinged,” “Shocking the Conscience,” “Paranoia and Pandemonium” and “Scare-a-Thon.” This verbiage makes Rucker and Leonnig’s book sound like one more enraged polemic. It isn’t. They’re meticulous journalists, and this taut and terrifying book is among the most closely observed accounts of Trump’s shambolic tenure in office to date.

Rucker is The Washington Post’s White House bureau chief; Leonnig is a national investigative reporter for the newspaper. Both have won Pulitzer Prizes. Their newspaper’s ominous, love-it-or-hate-it motto is “Democracy Dies in Darkness.” “A Very Stable Genius” flicks the lights on from its first pages.

Rucker and Leonnig have composed their book, they write, out of a desire to step out of the churning news cycle and “assess the reverberations” of Trump’s presidency. The result is a chronological account of the past three years in Washington, based on interviews with more than 200 sources.

It reads like a horror story, an almost comic immorality tale. It’s as if the president, as patient zero, had bitten an aide and slowly, bite by bite, an entire nation had lost its wits and its compass.

The result of Rucker and Leonnig’s hard work is a book that runs low to the ground; it only rarely pauses for sweeping, drone-level vistas and injections of historical perspective. This is not Garry Wills or Joan Didion. They do break news, some large and some small.

An example of large news: They report that in the spring of 2017, Trump implored Rex Tillerson, then secretary of state, to help him jettison the Foreign Corrupt Practices Act. “It’s just so unfair that American companies aren’t allowed to pay bribes to get business overseas,” Trump whines to a group of aides. Nearly every line from Trump, in “A Very Stable Genius,” is this venal.

Another example: While visiting Pearl Harbor, according to John F. Kelly, Trump’s former chief of staff, Trump seemed to have no idea what had actually happened there. Throughout he is misinformed and confused while at the same time utterly certain of himself.

An example of small but nonetheless piquant news: Administration lawyers came up with a nickname for Matthew Whitaker, the former acting attorney general. They called him Mongo, after the illiterate galoot played by Alex Karras in the Mel Brooks movie “Blazing Saddles.”

Rucker and Leonnig are adept at scene-setting, at subtly thickening the historical record. More than a few of these scenes feature Ivanka Trump and Jared Kushner, known to nearly all in the White House as “the kids.” They’re viewed as in over their heads and possessed of unfailingly defective judgment.

There’s a brutal scene early on, during the initial staffing of Trump’s White House, concerning Michael Flynn, the president’s first national security adviser. Despite warnings about Flynn — the authors describe his “Islamophobic rhetoric, coziness with Russia and other foreign adversaries and a reliance on flimsy facts and dubious assertions” — Trump’s team made it clear he could have any job he wanted in the administration.

The authors write: “‘Oh, General Flynn, how loyal you’ve been to my father,’ Ivanka said in her distinctive breathy voice, adding something to the effect of ‘What do you want to do?’ “

Before Trump had met with NATO allies, he kept glancing at Reince Priebus and pleading in front of others, in fanboy tones, “When can I meet with Putin? Can I meet with him before the inaugural ceremony?”

The authors build several stirring scenes around Tillerson’s experiences as secretary of state, and the disturbing behavior he witnessed. They provide the fullest picture to date of a now notorious July 2017 meeting in “the Tank” of the Pentagon during which military leaders and Trump’s national security team, alarmed by “gaping holes in the president’s knowledge of history and the alliances forged in the wake of World War II,” tried to give him a gentle lesson on American power.

The meeting ended after Trump exploded, saying, among other things, “You’re all losers, you don’t know how to win anymore,” and “You’re a bunch of dopes and babies.”

At a later meeting in the White House Situation Room, Trump began speaking, not for the first time, about his desire to make a profit from the deployment of American soldiers. Tillerson had finally had enough. The authors describe the moment. The secretary of state stood, facing away from the president and toward officers and aides in the room.

“I’ve never put on a uniform, but I know this,” the authors quote Tillerson saying. “Every person who has put on a uniform, the people in this room, they don’t do it to make a buck. They did it for their country, to protect us. I want everyone to be clear about how much we as a country value their service.”

Trump grew red in the face, but saved his fire for later. The chairman of the Joint Chiefs of Staff later called Tillerson, his voice unsteady with emotion, to thank him. There aren’t a lot of moments in “A Very Stable Genius” in which people do the right instead of the expedient thing. Some of these scenes may make the reader unsteady with emotion as well.

There’s a lot more here, amid the peeling wallpaper of the American experiment. Trump considered awarding himself the Medal of Freedom. He informed the Indian prime minister Narendra Modi, wrongly: “It’s not like you’ve got China on your border.” Photographs emerged of Colbie Holderness, one of the former White House staff secretary Rob Porter’s ex-wives, with a black eye. She said Porter gave it to her. Trump had another explanation. The authors write: “Maybe, Trump said, Holderness purposefully ran into a refrigerator to give herself bruises and try to get money out of Porter?”

There are grainy details of a physical altercation between Kelly and Corey Lewandowski. There is the belittling, by nearly everyone, of the acting chief of staff Mick Mulvaney (“He’ll take whatever I offer him”; “Mick just wants to be liked.”). There is commentary about Robert Mueller’s failure to press the president for a face-to-face interview during his investigation. There are descriptions of Mike Pence as “a wax museum guy,” able to blithely absorb any amount of insanity without comment.

In his memoir “A Good Life: Newspapering and Other Adventures,” the Washington Post editor Ben Bradlee wrote that Richard Nixon’s press secretary, Ron Ziegler, was “a small-bore man, over his head, and riding a bad horse.”

These words apply, one thinks while reading Rucker and Leonnig’s more than competent book, to nearly every adviser and staffer now in Trump’s orbit. The authors write: “The ineptitude came from the very top.”

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‘A Very Stable Genius: Donald J. Trump’s Testing of America’

By Philip Rucker and Carol Leonnig

465 pages. Penguin Press. $30.

© 2020 The New York Times Company