To clarify his perspective on all of that “messiness and unpleasantness and awkwardness,” Geithner has spent the past year writing a book — “Stress Test: Reflections on Financial Crises” — that will be published on May 12. It is filled with revealing and sometimes gripping behind-the-scenes anecdotes. Geithner acknowledges for the first time, for instance, that he was initially at odds with Paulson and Bernanke over whether to bail out Lehman Brothers in advance of the famous weekend meeting in which they sought to find a solution before the firm collapsed. (“I sensed their advisers pulling them toward political expedience,” he writes, “trying to distance them from the unpalatable moves we had made and the even less palatable moves I thought we’d have to make soon.”)

Geithner also discloses that some members of the administration talked openly about nationalizing some banks like Citigroup. (“If you want to go in, you better be sure there are W.M.D.'s,” Lee Sachs, an economic adviser, said during a meeting with Obama in the Roosevelt Room.) And Geithner discloses that he refused to fire Ken Lewis, the chief executive of Bank of America, who was near retirement. (“Tim, I’m trying to look out for you,” Geithner reports Gene Sperling, a counselor to him at the time, saying that he didn’t owe Lewis any favors. “If he’s going anyway, why don’t you push him out?”) He even concedes that he and Summers were initially opposed to the Volcker Rule, the widely popular regulation barring commercial banks from proprietary trading. His support, he writes, was “certainly political.”

But “Stress Test” is also surprisingly personal. Geithner confides that he actually didn’t want the Treasury job in the first place and that he tried on multiple occasions to resign, but Obama wouldn’t “liberate” him. He is also forthcoming — albeit in the somewhat unemotional language of the technocrat — about his own regrets. (“Before the crisis, I didn’t push for the Fed in Washington to strengthen the safeguards for banks, nor did I push for legislation in Congress to extend the safeguards to nonbanks,” he writes. “I wish we had expanded our housing programs earlier, to relieve more pain for homeowners.”)

Geithner likes to say that all the criticism and the second-guessing and the vitriol directed his way never got to him. “I try to pay no attention to that,” he told me over lunch one afternoon in New York. Almost indignantly, he added, “Our job was to fix it, not to make people like us.” Later, though, he softened and qualified the statement. “I’m human, and I like to be liked,” he said, “even if I didn’t expect to be liked in this.”

Geithner has remained silent about his time at Treasury, but over the past month, he and I met several times to discuss and debate his tenure. Despite the wonky monotone he often projected during the worst moments of the crisis, he is lively and quick-witted in person, and he has a special proclivity for particularly graphic language. Over lunch at Café Centro, near Grand Central Terminal, he told me a story from a few months earlier: “I was crossing Lexington Avenue and some guy said, ‘You’re one of the Goldman [expletive] who ruined the country.’ ” Geithner said he replied, “Thanks for sharing.” At another point, he cheerfully relayed a story that also appears in his book about the time he sought advice from Bill Clinton on how to pursue a more populist strategy: “You could take Lloyd Blankfein into a dark alley,” Clinton said, “and slit his throat, and it would satisfy them for about two days. Then the blood lust would rise again.”

Little of this personality emerged when Geithner was in office. A career public servant, whose work often took place behind the scenes, he seemed an unlikely fit for a high-profile position in the Washington fishbowl culture. When his name was floated for the Treasury job in the fall of 2008, he was busy helping stave off, as the president of the New York Fed, the collapse of Citigroup. Geithner had moved for work before, during the 1990s — from Washington to Tokyo and back to Washington for jobs at the Treasury Department — and now wanted to stay put in Larchmont, N.Y., a comfortable suburb a half-hour from Manhattan. “I felt guilty that I was even thinking about relocating again,” he writes in his book. “I had also promised Carole” — his wife — “that we would never again live apart, not for anything, and I knew we couldn’t move the kids in the middle of the school year.” Carole Geithner, a social worker and author, was openly resistant to the idea of her husband’s becoming the next Treasury secretary. “Carole didn’t just have reservations,” Geithner writes. “She was opposed.”