Fred Siegel writing on Rudolph Giuliani is a match made in heaven. Both men exemplify the feisty, independent spirit of New Yorkers at their best. I say “at their best.” For what Mr. Siegel makes clear in his book – clearer indeed than any writer has ever done in a book – is also something that Giuliani made clearer to the people than any mayor had done: New Yorkers often mire themselves in interest-group and identity politics and behaviors that have drained the city of so much of its creativity, and adaptability – more than should ever, ever have been the case.

New York once stood poised to be the great American city, as Paris is the great French city and London the great English city. But while New York remains the largest American city, and preeminent in many areas, still its stature and status vis-a-vis other U.S. localities has fallen precipitously in the decades since World War II. What happened forms the first 100 or so pages of Mr. Siegel’s new book, “The Prince of the City: Giuliani, New York, and the Genius of American Life” (Encounter Books, 320 pages, $26.95).

Those looking for a biography of Mr. Giuliani will be disappointed. This is a work of urban history, rich in detail on the warp and woof of New York life in the Giuliani years. Mr. Siegel recounts the mayoralties of Wagner, Lindsay, Beame, Koch, and, especially, Dinkins, as he traces the city’s fall from grace – the municipal profligacy, the unhinged unions, the fiscal shenanigans, the willful racial polarization, the “deferred maintenance,” the rampant criminality – and paints a portrait of the city that Rudolph W. Giuliani inherited upon assuming City Hall in January 1994.

It was a city and a state in which the New Deal “took root more deeply … than almost anywhere else. “The interest-group politics the New Deal unleashed were, Mr. Siegel writes, “kept in check in the 1930s, 1940s, and 1950s by the great solidaristic efforts at combating Depression and war. But by the 1960s and the governorship of Nelson Rockefeller, the tendency of powerful organized interests to raid the treasury at the expense of the unorganized sent the economy aground.”

Describing the modus operandi of Governor Cuomo, “the philosopher-king of the Democrat Party” in the early 1990s recession, Mr. Siegel writes: “In a state where the largest lobbyists were almost all from the public sector, broadly defined, the legislature used the taxpayer’s money to insulate the key interest groups associated with each legislative chamber from the effects of the recession. It didn’t matter that the pie was shrinking so long as the interest groups were able to grab a larger piece of what was left.”

Mr. Siegel is excellent – the best I’ve ever read – on why Mr. Giuliani nonetheless supported Mr. Cuomo over George Pataki in the 1994 gubernatorial race, in spite of Mr. Cuomo’s standing for everything Mr. Giuliani distrusted in interest-group politics. New York politics aren’t easy, and, in the Machiavellian vein indicated by the title of Mr. Siegel’s book, Mr. Giuliani’s endorsement proved a political masterstroke. He procured a “deal” for state approval (via the Cuomo-controlled Municipal Assistance Corporation) to the city to offer severance packages to bring municipal employment back down to the level preceding Mayor Koch’s hiring binge, which had set the city on the perilous fiscal course only worsened in the Dinkins years. He also won a new respect among many of the city’s African-Americans, who adored Mr. Cuomo.

Mr. Siegel is also excellent on the crime-fighting techniques put in place under Mr. Giuliani’s first, brilliant police commissioner, William Bratton – the combination of “broken windows” policing and the computerized analysis of crime data known as “CompStat” (the latter the brainchild of Jack Maple). In 1994, Mr. Giuliani’s first year in office, crime dropped 12%; in 1995 it dropped 16%, as it did again in 1996. The greatest drops were in the most crime-ridden outer-borough neighborhoods, a point often ignored by Mr. Giuliani’s critics.

Mr. Siegel is equally concise, clear, and solid on Mr. Giuliani’s efforts at welfare reform, and on his crackdowns on the influence of organized crime in New York business – the so-called “mob tax.” He then examines Mr. Giuliani’s strained relations with the press, and his unfortunate fallout with Mr. Bratton.

When all is said and done, Mr. Giuliani’s tenure as New York’s mayor will be remembered for two things. First is his leadership in the wake of September 11, 2001. Second is his spearheading of the most dramatic crime reductions the city had ever seen, reductions far out stripping those that occurred in other American cities (again, a point often ignored by the mayor’s critics). But only those, like Mr. Siegel or me, who lived through both the before and the after can truly appreciate what this crime reduction did for the city’s psyche.

In Mr. Siegel’s words: “As fear declined even more rapidly than crime, the effect on daily life was palpable. A virtuous cycle was set in motion in which people spent more time in public places, and, as good uses of public space drove out bad, more people were drawn back into the public life of the city.”(Mr. Siegel, by the way, in saying “good uses drive out bad,” echoes the famed urbanist, the late William H. Whyte, whom Mr. Giuliani called in as an adviser on public spaces.) I encounter far too many people, though, who lived through the high-crime times who have simply chosen, out of an ideological hatred of Mr. Giuliani, to forget how rotten it was. I haven’t forgotten. And I never will.

Mr. Giuliani’s second term was not as successful as his first. When he began it, in January 1998, it was hard for many to imagine, with the dot-com boom in full swing, crime down, and so on, that the city had ever been better. The national media, through such shows as “Seinfeld” and “Friends,” presented an image of New York as a big small town peopled by lovable eccentrics, and young people flocked to the city in a way they had not in years, if ever. Mr. Giuliani was not personally responsible for all of this, but, remarkably, he was responsible for an awful lot of it – he had conquered New York.

The second term, however, brought derision over his strange efforts to realign pedestrian crossings in Midtown, and to crack down on jaywalkers. When he tried to start a movement for greater civility, he was jeered off the stage by the likes of Jimmy Breslin, who applauded New Yorkers’ bad manners while failing to realize of what recent vintage the bad manners were and how little they had to do with New York in its most characteristic historical moments. And it was all nothing compared to the shooting of Amadou Diallo, which, as Fernando Ferrer could tell you, still plays very big in New York politics. Then there was the Brooklyn Museum fiasco, and the divorce. It was a pretty rough second term.Toward the end, it seemed almost as though the public was tiring of Mr. Giuliani as it had tired of the once immensely popular Ed Koch.

Then came September 11, 2001.

Mr. Siegel is one of the most independent-minded urban commentators in America. A man of broadly social democratic sympathies who has never underestimated the power of conservative ideas, he writes from a position of great moral as well as intellectual authority. Ultimately, his most important point is that there are far too many people in New York – whether government or parts of the government-supported nonprofit sector – who may profit from the city’s slow death. The case of Rudy Giuliani is instructive in two ways: It shows both that a man of political courage, determination, and great intelligence can change the system, and how hard it is for such a man not to be defeated by it.