“Look,” a genial Eliot Spitzer says in one of his campaign ads beaming across the Empire State, “I had a simple rule. I never asked if a case was popular or unpopular. Never asked if it was big or small, hard or easy. I simply asked if it was right or wrong. In the end, it’s not a bad rule.”

No, it’s not a bad rule for a politician or any upright citizen to follow, but was it the guiding principle of Mr. Spitzer’s two terms as attorney general of New York? The answer, according to Brooke Masters’s engrossing but ultimately frustrating narrative of Mr. Spitzer’s rise to fame, “Spoiling for a Fight: The Rise of Eliot Spitzer” (Times Books, 368 pages, $26), is far more complicated than the man favored to be the next governor of New York would have voters believe.

Ms. Masters, a veteran Washington Post reporter based in New York, marshals considerable evidence to bring to life two different Eliot Spitzers. One is a leader who decorated his Lower Manhattan office with a framed photograph of President Theodore Roosevelt, whose Progressive movement against corporate greed inspired Mr. Spitzer’s own campaign to restore marketplace integrity amid a new age of excess. This is the Mr. Spitzer who claimed the mantle of federalism by filling a regulatory void created by Republican control of the White House.

The other Eliot Spitzer is a prosecutor who exploited election finance loopholes to bankroll his 1994 and 1998 campaigns, took advantage of the economic vulnerability of corporations, seethed at federal regulators when they stepped into his spotlight, decided whom to handcuff based on a calculus of political expediency, and who, as one of his critics described it, removed a cancer but sacrificed a lot of healthy tissue. While these two Spitzers never do coalesce into a realized portrait of a man rocketing to the apex of his career, Ms. Masters, with strenuous objectivity allows readers to make up their own minds.

Though promoted as the first biography of Mr. Spitzer, “Spoiling for a Fight” functions best as a precise reconstruction of the still-smoldering legal battles the attorney general waged against investment banks, mutual funds, the insurance industry, and a former New York Stock Exchange chairman, Richard Grasso.

Ms. Masters includes a chapter of tidbits culled from interviews with family members and friends (his father, a wealthy real estate developer, drove him to tears during a cutthroat game of Monopoly), his days as a grind at Horace Mann, Princeton University (he was elected student government president as a sophomore, wrote a 147-page term paper on post-Stalin Eastern Europe in a mere two months, and romanced Anne-Marie Slaughter), and Harvard Law School, and his stint at the Manhattan district attorney’s office, where he used 19th-century antitrust statutes to break the mob’s grip on the garment industry.

But Ms. Masters is not concerned with Mr. Spitzer’s personal life. Nor is she intent on charting his political future. Her attempts to speculate about what sort of governor Mr. Spitzer would be are cursory, amounting to a few paragraphs about how the attorney general’s strategy of using the press and making his battles public would clash with Albany’s culture of secrecy.

The chief focus of her book is unraveling the cases that made Mr. Spitzer famous. Using dozens of interviews with Mr. Spitzer, his deputies, defense lawyers, and executives, Ms. Masters pieces together the high-profile investigations from tip to settlement. The result sheds much light on the inner operations of the attorney general’s office and the industry secrets and myriad conflicts of interest that were the subject of the probes. We learn, for instance, how an anonymous tip from a nervous investment fund manager named Noreen Harrington led to an investigation into the practices of market timing and after-hours trading that allowed hedge funds to skim profits off mutual funds. It’s first-rate explanatory journalism that rarely loses the reader despite the complex subject matter.

Ms. Masters also is interested in pinning down motive and strategy, areas where Mr. Spitzer’s “simple rule” doctrine takes the hardest hit. After a judge tossed out Mr. Spitzer’s public nuisance lawsuit against the gun industry, the attorney general and his staff came to the conclusion that “public shaming worked much better when their opponents were economically vulnerable,” Ms. Masters writes, offering insight into one key reason why Mr. Spitzer homed in on Wall Street, which avoids bad publicity like kryptonite. Shaming was a crucial tactic for the attorney general; when his office wanted a plea deal from a Millennium Partners executive, Steven Markovitz, who was accused of after-hours trading, it gave him 24 hours to confess or “they would arrest him at home in front of his child and pregnant wife,” Ms. Masters writes.

The author also depicts an attorney general who guarded his investigations from the SEC like a journalist protecting his scoop, and who vied with the regulatory body for press attention. Days after the SEC brought an action against portfolio managers at Putnam Investments in 2003, Mr. Spitzer crowed that he had uncovered “one of the most scurrilous tales I’ve seen yet,” disclosing details about a pending investigation. He then castigated the SEC, telling the New York Times: “Heads should roll.There is a whole division at the SEC that is supposed to be looking at mutual funds. Where have they been?” The leak and attack worked, Ms. Masters writes, putting Mr. Spitzer back in the headlines.

While Ms. Masters’s efforts to mute her own opinions result in an admirably balanced book, some of the hesaid, she-said material begs for an umpire to make a call. She spends pages detailing Mr. Spitzer’s efforts to dig up proof of market timing and late trading but concedes in a sentence that the impact of the illegal practices on investor returns was “perhaps hundredths of a percentage point a year.” So, what was the fuss all about? It’s also unclear the extent to which the structural changes imposed by Mr. Spitzer actually are saving investors money. Most of the settlement money was sucked into the black hole of the New York treasury, leaving one to wonder what the small investor gained.

“Spoiling for a Fight” also glides over the tougher decisions Mr. Spitzer had to make. One was his choice to leave Carl McCall, the 2002 Democratic candidate for governor, out of the complaint against Mr. Grasso over his compensation package, despite allegations that Mr. McCall had failed to share information with compensation committee members about $48 million in future payout money to Mr. Grasso. The Wall Street Journal and other news outlets accused Mr. Spitzer of giving Mr. McCall a pass for political reasons, while Mr. Spitzer argued that the evidence against him was not strong enough. It would be interesting to know if there’s any internal e-mail correspondence among staffers in Mr. Spitzer’s office about the McCall decision. It’s not clear if Ms. Masters bothered to use New York’s powerful Freedom of Information Law to request documents related to the matter that could answer lingering questions.

After all, one could argue that Mr. Spitzer became famous by exposing other people’s e-mail.

jacob@nysun.com