David Dorsen makes no attempt to hide the conclusion of his new book "Henry Friendly: Greatest Judge of His Era," a biography of the legendary member of the U.S. Court of Appeals for the 2nd Circuit in New York from 1959 until his death at age 82 in 1986. The biography's title is no exaggeration. Friendly's reputation and résumé still glitter. Richard Posner, who's widely viewed as the greatest federal judge of the last generation, writes in the introduction to Dorsen's biography that Friendly was "the most powerful legal reasoner in American legal history."

Even the U.S. Supreme Court turned to Friendly for guidance. According to former Justice Sandra Day O'Connor, "When we are looking for a Court of Appeals decision for use as authority, we look first for opinions of Henry Friendly," and opinions from the high court have cited Friendly far more than any 20th-century judge except Learned Hand, who sat on the federal bench for 52 years, as compared with Friendly's 27.

Dorsen illustrates that point by marching through Friendly's body of work legal discipline by legal discipline, from the First Amendment to administrative law, an approach that can wear on the nonspecialist. But Dorsen's biographical sketch offers some fascinating pieces of American legal history, and Posner's introduction is a valuable evaluation of Friendly by a fellow judge.

Born in Elmira, N.Y., in 1903 to a well-off German-Jewish family, Friendly was a brilliant student of medieval history at Harvard College, but he went to law school rather than graduate school after a talk with Felix Frankfurter, then a law professor at Harvard and later a justice on the Supreme Court. Friendly had one of the best student records ever achieved at Harvard Law, from which he graduated in 1927, and Frankfurter secured him a clerkship with Justice Louis Brandeis.

Friendly could have returned to Harvard or gone into government. But he wanted to make money, so he opted to join Root, Clark, Buckner, Howland & Ballantine, one of the few elite firms in New York to hire Jews. Friendly chose Root Clark over an offer from Sullivan & Cromwell LLP; he was unaware that three of the firm's partners were Jewish in an era when law firms were tiny.

Shortly after joining Root Clark, Friendly began doing work for Pan American Airways, which became his most important client. He made partner in 1937 but split off with a group of other partners in 1945 to form Cleary, Gottlieb, Friendly & Cox, the forerunner of today's Cleary Gottlieb Steen & Hamilton LLP. Friendly began to tire of law firm life in the early 1950s, and his sterling academic record at Harvard made him a prime candidate for a seat on the bench, to which he was appointed in 1959. He was sworn in by John Marshall Harlan, a close friend from Root Clark who in 1955 became a justice on the Supreme Court.

Friendly took to the job immediately. Six months into his tenure, fellow 2nd Circuit Judge Learned Hand wrote to Frankfurter, "Friendly is realizing all our hopes." Clerkships for Friendly were highly coveted, and 19 of his 51 clerks, including John Roberts Jr., went on to clerk for Supreme Court justices.

For all of that, Friendly isn't associated with a single doctrine or groundbreaking decision. "Friendly had no agenda," Dorsen writes. "He tended to shy away from seemingly bright-line rules. Indeed, part of his brilliance was recognizing that there were no easy answers to the problems that confronted him." In a review of Dorsen's book that ran last month in The New Republic, Harvard Law professor Adrian Vermeule argued that Friendly's lack of "articulate and exportable ideas" relegates him to secondary status in the American judicial firmament.

Friendly won the reverence of his peers and successors not by reimagining entire fields of law but by striking a balance between respect for precedent and intellectual creativity that proves to be almost impossible to achieve. He was masterful at presenting the facts in a case, and, Posner writes, he "did not feel himself bound by the issues as framed by lawyers. Friendly didn't just decide interesting cases; he made interesting cases," Posner writes. "He thus was something of a judicial buccaneer -- a role not to be recommended to the average judge." Friendly thus merits study not as a model for other judges but as a rare example of legal genius.

David Marcus covers legal matters for The Deal magazine.