On Easter Sunday, Chris Christie, the former New Jersey governor, was holding forth in the green room at ABC News in New York before going on the air. A friend of Donald Trump from well before the 2016 campaign, Christie had run against Trump in the Republican primaries, then dropped out and travelled around the country on behalf of his onetime rival. Eventually, he went on to become the chief of Trump’s transition team, although, when Trump surprised everyone by winning the election, Christie was pushed out in favor of Vice-President-elect Mike Pence. These days, Christie is once again a regular on Trump’s call list, one of the outside advisers with whom the President frequently consults in lieu of a more conventional White House staff. Trump has even been known to call and kibbitz with Christie over Sunday-show appearances like the one he was about to make.

In other words, Christie knows Trump a lot better than the rest of us do. As I and a couple of other guests peppered Christie that morning at ABC with questions about how to understand Trump (the latest tumult included his firing, by tweet, of the Secretary of State, ousting the Veterans Affairs Secretary, and deciding to name the unvetted White House physician to the post instead), he interrupted in frustration. Christie argued that pesky journalists and amateur Trump watchers were always getting the President wrong, making it out as if there were some “Machiavellian” grand plan by Trump that could explain many otherwise seemingly unexplainable moves. “There is no strategy,” he exclaimed.

Christie may not be the White House chief of staff, or Attorney General, or any of the other jobs he once aspired to in a Trump Administration (and his prospects remain dim, so long as his rival Jared Kushner, the President’s son-in-law, stays in the White House; Christie, as a federal prosecutor, helped send Kushner’s father, Charles, to jail). But, as far as I’m concerned, he belongs on the short list of reliable Trump decoders, at least when he avoids partisan spin and transparently self-serving plugs. Others have their own favored Trump whisperers helping to make sense of this President. Administrations have always needed their interpreters to the outside world, but never more than this one, where Trump explodes orderly decision-making processes, routinely countermands or bypasses his staff, and just generally operates as if his is a government of one. “I’m the only one that matters,” he proclaimed in response to a question, last fall, about the many vacancies in the State Department. Increasingly, the evidence suggests that is true when it comes to consequential decisions, such as this week’s move to withdraw the United States from the Iran nuclear agreement. For months, unhappy U.S. allies fruitlessly tried to persuade him to stay in the deal even though savvy observers knew, all along, that Trump, who called it the “worst deal ever” on the campaign trail, was determined to blow it up.

Given Washington’s new Trumpian reality, I asked more than a dozen of my favorite reporters and analysts whom they turn to for help in figuring out the President. Several mentioned White House aides or outside advisers, such as Christie, who seem to have the ability to read Trump’s quirks and offer reliable guidance about what a President who delights in the appearance of unpredictability will actually do. One White House reporter, for example, said that Stephen Miller, the combative young aide who writes many of Trump’s speeches and has helped shape his hard-line immigration policy, was “an authentic reflection of his boss,” citing him as a helpful resource “if you’re looking to decode what Trump is really thinking.” Others mentioned informal advisers including Newt Gingrich, the former House Speaker, and Christopher Ruddy, the C.E.O. of Newsmax and another regular Trump phone buddy; journalists such as the Times’ Maggie Haberman and the Free Beacon’s Matthew Continetti; and Trump himself, with his prolific Twitter feed and unfiltered, rambling public monologues. “The truth is, virtually everyone who claims to know what Trump is going to do has been wrong at some point,” one sharp analyst told me. “The best indicator, in my mind, is to go back and read his core campaign pledges and speeches. Those have been far more instructive than anyone in Congress, in the Republican Party, or on his own team.”

Many of those whose job it is to understand Trump believe, as the Washington Post White House bureau chief, Philip Rucker, put it, that “the Donald Trump of today is the same Donald Trump of decades past, so to decode his moves as President, I find it especially instructive to talk to his biographers for insights into his actions and characteristics before he took office.” Among those he and others named were Timothy L. O’Brien, the author of “TrumpNation,” and Tony Schwartz, the ghostwriter of “The Art of the Deal,” the book that made Trump a household name. Another White House correspondent recommended “Trumped!,” a tell-all by one of Trump’s former casino executives, Jack O’Donnell. “All the same traits repeat themselves now,” the correspondent wrote to me. “The grandiosity, the impatience and impulsiveness, the repeated lies. He observed Trump up close for three years and writes with more honesty and sharper observational powers than anyone else who had that kind of sustained proximity to Trump over the years.” I agree that the Trump biographers are an invaluable resource; as Trump was surging toward the Republican nomination, in the spring of 2016, I convened the authors of the five books on him for a Politico Magazine roundtable at the Trump Grill, in Trump Tower. I half-jokingly christened the project “Trumpology,” and another convening of the group on the eve of Trump’s Inauguration was an uncanny guide to what would soon transpire in Trump’s White House, from the obsession with “loyalty” on his staff to his refusal to hew to the established norms of the office. At the time, other observers, less schooled in Trump, wrongly thought that the heavy responsibilities of a job for which he was ill-prepared might change him. Not the Trumpologists. “He’s the same old Trump,” Gwenda Blair, the author of “The Trumps: Three Generations that Built an Empire,” said back then. She was right.

As a practical matter, those who for years have observed Trump up close remain the best guides to his Presidency, even if they are not at all plugged into the day-in, day-out battles over personnel and policy that consume his White House. One of the best of today’s journalistic chroniclers of Trump reinforced that point in answer to my query. “The three people who understand him best outside his family are Michael Cohen, Roger Stone, and Sam Nunberg,” this journalist responded, referring to the President’s now embattled personal lawyer Cohen; his on-again, off-again friend of decades Stone; and his 2016 campaign aide Nunberg. “For me, they helped inform my thinking over the years more than anyone.”

Cohen, it turns out, has also learned more than anyone how to profit from his skills as a Trumpologist. The President’s lawyer and designated fixer appears to have been selling this analysis to others, a fact we know now courtesy of Stormy Daniels, the porn actress whose alleged affair with Trump Cohen had been working to cover up. On Tuesday, Daniels’s lawyer, Michael Avenatti, released a dossier purporting to show that Cohen had received six- and seven-figure payments from several large corporations and the U.S. affiliate of a Russian oligarch. After the news broke, most coverage, understandably, revolved around the purported Russia connection: Was this, at last, an actual financial link between Russia and the President who has denied having any? But I was also struck by the subsidiary scandal of White House access-selling that the documents seemed to disclose, especially once the corporations started confirming they really had been paying the President’s longtime confidant. In its statement, for example, the telecommunications giant A.T. & T. acknowledged that it had signed a contract with Cohen for hundreds of thousands of dollars soon after Trump’s Inauguration; no lawyering or lobbying was involved, the company said, and Cohen’s only job was to “provide insights into the new administration.” Trumpology, it seems, never paid so well.