In 1994, legendary Washington private eye Terry Lenzner did the first deep-dive into Bain, the firm stewarded by Mitt Romney.

‘Even sophisticated people who should have known better,” writes Terry Lenzner, “can do some pretty stupid things.” He should know: The instinct for the self-inflicted wound that beats within the breast of the rich and powerful has made the Harvard-trained lawyer and former federal prosecutor, now the chairman of a private-investigation firm, rich and celebrated in his own right. In “The Investigator” he tells his life story so far.

Though Mr. Lenzner is best known for his role as an assistant chief counsel to the Democratic majority on the Senate Watergate Committee and for his work on President Bill Clinton’s defense team during the Lewinsky scandal—when he was tasked with digging up dirt on the president’s accusers—Mr. Lenzner has enjoyed a varied career that stretches from the civil-rights movement to the present day. Zelig-like, he keeps popping up at the various intersection points of politics, journalism and the law.

Detailed yet accessible, “The Investigator” presents a breezy tour of postwar America by a relentless pursuer of concealed facts, a hired gun whose services can be purchased but whose allegiances cannot. This independent streak leaves Mr. Lenzner—a native New Yorker raised by quarrelsome parents who, he tells us, never embraced their Jewishness—a highly skeptical character. Accordingly, he can be refreshingly iconoclastic in his views of the people and events he has observed up close.

John Dean, for instance, the White House counsel who testified against President Richard Nixon and his aides at the Watergate hearings, is described by Mr. Lenzner as an “oversung” hero. “In my first encounter with him, Dean seemed more intent on bragging about the number of women he’d attracted than in offering high-minded views of the appropriate use of executive power,” he writes. “The main narrative in the media soon became that Dean was a noble ‘man of conscience’ who turned against his former boss and sacrificed his high position to expose wrongdoing. I never saw him that way.”

While Mr. Lenzner shares the conventional view of Independent Counsel Kenneth Starr as a prosecutor who badly overreached when he pursued Mr. Clinton for lying under oath about his affair with White House intern Monica Lewinsky, Mr. Lenzner makes it clear that he is no fan of the Clintons. He argues that Mr. Starr would have fared better if he had narrowed his focus to “Clinton’s foreign money contributions” in the 1996 campaign. “I was amazed,” he writes, “that the Clinton White House managed to get away with receiving hundreds of thousands of dollars in illegal foreign money—including from a guy [Arkansas restaurateur and fundraiser Charlie Trie ] who knew Clinton when he was governor.”

One irony of “The Investigator” is that the chapter on the Watergate scandal is among its least compelling. To support the claim that Nixon ordered the break-in, Mr. Lenzner resuscitates the so-called Howard Hughes theory. Popular in the 1970s, this theory posited that the telephone of Larry O’Brien was tapped because the Democratic Party chairman maintained a secret consulting contract with the reclusive industrialist and Nixon was determined to find out what O’Brien might know about an illegal $100,000 cash payment that Hughes had made to a Nixon confidant in 1970, designed to buy laxity in the enforcement of antitrust laws.

Two critical facts hobble this theory. First, the evidence strongly suggests that Nixon didn’t order the break-in; the president’s tapes captured his surprise and anger at learning of it. Second, the wiretap installed on O’Brien’s phone was never operational and thus never monitored. The Watergate team spent three weeks monitoring, instead, the telephone of a secretary who worked for a DNC official named R. Spencer Oliver Jr. More recent scholarship has produced a plausible theory as to why: Mr. Oliver’s telephone was purportedly being used to set up dates between Democratic Party officials and a call-girl ring near the Watergate. Readers of Mr. Lenzner’s book might not know of these facts because the celebrated investigator never mentions them.

Far more engrossing are the book’s tales of Mr. Lenzner’s early days as a Justice Department attorney, braving the Ku Klux Klan in the segregated South. Pounding the pavement in terrified communities, Mr. Lenzner helped the Justice Department’s Civil Rights Division identify perpetrators in the 1964 “Mississippi Burning” case, when the murder of three civil-rights workers galvanized the nation, and in the 1965 “Bloody Sunday” case, when racist lawmen attacked nonviolent marchers at the Pettus Bridge in Selma, Ala.

Other investigative tales are gripping in different ways. After being retained by the 1994 re-election campaign of Sen. Edward Kennedy, Mr. Lenzner

did the first deep-dive into the history and conduct of Bain Capital, the financial firm stewarded by Kennedy’s GOP opponent that

year: Mitt Romney. As Barack Obama can attest, the early spadework Mr. Lenzner did on Bain would prove useful to Democratic operatives for decades to come. Certainly Mr. Romney was impressed: For his winning 2002 gubernatorial campaign in Massachusetts, he retained Mr. Lenzner’s firm. With touching sensitivity, Mr. Lenzner also relates the story of how Mohamed al-Fayed —the father of Princess Diana’s lover, Dodi, killed in the car crash that also took her life—retained his firm, in vain, to corroborate the conspiracy theories that Mr. al-Fayed nurtured as a way of assuaging his own grief.

Mr. Lenzner is no prose stylist, but eloquence was never his calling card: He has always trafficked in data, before anyone called it that. So decisive have been his victories that those who have encountered him are divisible into two groups: those who thanked God he was on their side and those who rued that he wasn’t.

Mr. Jay Rosen is chief Washington correspondent for Fox News and author of “The Strong Man: John Mitchell and the Secrets of Watergate.”