(FORTUNE Magazine) -- In 1933, the U.S. Senate passed a bill mandating a 30-hour workweek. Alabama Senator Hugo Black, still a few years from the Supreme Court, was the sponsor.

This now seems remarkable in many ways: Who knew that we were so far ahead of the French? That Alabama used to elect union-friendly radicals? That Hugo Black was one of them?

Jonathan Alter brings up the Black bill in The Defining Moment: FDR's Hundred Days and the Triumph of Hope for another reason.

It was an example, one of the many that Alter invokes, of the way things might have gone if it hadn't been for the country's leader, Franklin Delano Roosevelt, who headed off Black's legislation in the House of Representatives.

In Alter's highly readable telling, the deeply depressed nation was headed in a very scary direction in 1933 when F.D.R. took office.

A complete economic unraveling appeared to be in the offing. Lots of smart people thought we were headed toward communism.

Others wanted Roosevelt to seize power from Congress and become a dictator. (Some conservative critics would argue that Roosevelt went on to do exactly that. Alter's point is that by any reasonable standard he did not, and that this in itself was an accomplishment.)

Alter writes that F.D.R. possessed "a short attention span, an eye for the spotlight, and a fierce ambition."

These seemingly ignoble attributes are exactly what made Roosevelt so effective: He was willing to try almost anything and to switch course quickly when an initiative sputtered. (His alternative to the Black bill, incidentally, an initiative called the National Recovery Administration, did sputter and was struck down by the Supreme Court in 1935.)

But F.D.R. had an innate understanding of how to use the media to his advantage, and he plowed right on through disasters that would rob most of us of all confidence.

Lots of Roosevelt's policy initiatives were misguided, and the nation didn't fully emerge from the Depression until World War II.

But in his first year in office Roosevelt restored hope (not to mention the solvency of the nation's banking system), and in the process he may have saved American democracy.

This is Newsweek columnist Alter's first book, and sometimes that shows.

Its constituent parts - biography, history, essay on leadership - don't always fit together, and it's odd that a book purportedly about F.D.R.'s hundred days devotes only 103 of its 337 narrative pages to them.

Alter is so danged smart and perceptive, though, that none of this matters much.

He is after a definition of what constitutes effective leadership, and along the way comes across lots of revealing surprises. Yes, critics were right in calling Roosevelt a dilettante and a spoiled mama's boy.

But those turned out to be strengths.