If Barack Obama feels saddled with high expectations, he should reflect on a letter that greeted Franklin D. Roosevelt in 1933. Wrote a woman in New Jersey, “I truly believe you have been sent directly by God to our nation, for such a time as this.”

Unimaginably, almost one in four workers was out of a job when F.D.R. became president. Banks had failed; the stock market had crashed; auto production had fallen by four-fifths. Soup kitchens could not feed the hungry, and across the country, even in New York City’s Central Park, families camped in tar-paper Hoovervilles. With the Republicans and their doctrine of laissez-faire having been routed, the people’s hunger for leadership was nearly as great as their desire for food. Watching the emotion of the crowd as her husband took the oath of office, ­Eleanor Roosevelt was uneasy: “You felt they would do anything—if only someone would tell them what to do.”

In the subsequent hundred days, F.D.R. enacted reforms that addressed agriculture, Wall Street, and industry, and undertook the obligation, heretofore unmet by the federal government, to provide for citizens in need of jobs, nourishment, and, yes, mortgage relief. This legislative blitzkrieg set the template for administrations confronted by an economic crisis.

As Adam Cohen, an assistant ­editorial-page editor at the New York Times, observes—in a sentence that seems as if it could have been lifted from the Obama campaign—the “pledge of a New Deal was a commitment to bring about change.” Cohen’s Nothing to Fear: F.D.R.’s Inner Circle and the Hundred Days That Created Modern America is a passionate and admiring look at this oft-examined historical crucible, of which Arthur Schlesinger Jr.’s The Coming of the New Deal, published in 1958, remains the standard study.

Perhaps the chief excitement in Nothing to Fear is the tantalizing hope that buried in the rich historical milieu the book considers lies a prescription for our present-day troubles. In a literal sense, this is unlikely, for as Cohen makes clear, F.D.R. did not enter office with any prescription or even a coherent agenda. The New Deal was a “hodgepodge,” a mash of policies related only by the loose intention of solving sundry economic problems. F.D.R. essentially proposed the remedies urged on him by his closest advisers, and if some of these counselors hotly disagreed with one another, it troubled the new president not one whit.

Henry Wallace, the Iowa-bred secretary of agriculture, spearheaded legislation to limit crop cultivation and lift farm prices. Frances Perkins, a student of urban blight, had crusaded for worker protections since the time of the Triangle Shirtwaist Factory fire, which she witnessed; as secretary of labor, she pushed for public works. Directly opposed was Lewis Douglas, the tightwad budget director. Thanks in part to Douglas, the New Deal began, oddly, with a draconian budget cut. Soon after that, Harry Hopkins, a social worker who had directed relief efforts in New York State, barged in on Perkins demanding an audience and was promptly named head of the Federal Emergency Relief Administration. Within two hours of starting, he had distributed $5 million to eight states that had run out of relief money.

Cohen’s decision to focus on F.D.R.’s advisers is consistent with their large and formative roles. (Perhaps it was also tactically motivated; books on F.D.R. himself are not exactly scarce.) And the advisers had remarkable careers. Perkins, who had to overcome the considerable scorn reserved for female administrators—she was the first woman to become a member of the cabinet—would spearhead Social Security, the New Deal’s most enduring program.

Cohen shows us just enough of the chief, who vacillated between his aides’ policies and trusting his own political antennae. Roosevelt was at heart a conservative—religious, disdainful of extravagance, and partial to the pastoral life of his childhood. His beloved Civilian Conservation Corps, which sent thousands of the unemployed to restore forest land, stemmed from the president’s love of nature, fused with his Protestant scorn for idleness. He was loath to provide relief lest it sap the ambition of workers. Owing to an analogous fear (echoed during 2008) that government protection of banks would dampen their incentive to scrutinize loans, F.D.R. opposed, though ultimately countenanced, the establishment of deposit insurance—the single reform that would prove most effective in preventing bank runs and depressions in the future.

Some of Roosevelt’s policies were stupendously ill-conceived, notably the National Industrial Recovery Act, which was designed to subject private industry to hundreds of government “codes.” (The Supreme Court eventually overturned it.) Yet the plan—or the un-plan—worked. His prejudices notwithstanding, F.D.R. sensed that the system was so wounded that only radical reform could save it. The New Deal didn’t cure the Depression, but it brightened the lives of millions who lived through it and set the economy on an upward plane.

By 1937, Cohen notes, economic output had returned to its 1929 level, and unemployment had eased to 14 percent. The economy would suffer a relapse, but F.D.R.’s essential innovation—to recognize the state’s role as guardian—survived.

Was there a method in all this? F.D.R.’s critics rebuked him for being inconsistent: a “chameleon on plaid.” So be it. He was confident enough to tolerate spontaneity and a free competition of ideas and thus breathed life into a stricken giant. My fear for the 44th president is that he governs in a much more choreographed age. We will see if Obama can break free of it.