After the war, he became Kentucky's attorney general and twice ran, unsuccessfully, for governor. He had no political home of consequence (before the war, he had belonged to the Know-Nothings; after, to the Conservative Unionists) until he joined up with the Republicans, renouncing many of his previous positions on race, slavery, and states' rights in order to do so. In 1877, he was nominated to the U.S. Supreme Court by Rutherford B. Hayes and (not without opposition) was confirmed.

The subject of Canellos' book isn't just one man, but also the incredibly strange, heartbreaking complexity of race relations in America during that man's lifetime. In order to demonstrate that complexity, Canellos has at hand another character, the ideal foil for his protagonist -- John's half-brother Robert Harlan.

They had the same white father, James Harlan, but Robert's mother was an enslaved person, and Robert, too, held that status in the Harlan household. The light-skinned Robert was educated by the other Harlan brothers and eventually allowed to buy his freedom. In what Canellos calls "the odd etiquette that surrounded the slave culture," Robert's parentage was assumed but never discussed:

"It was a form of ritual denial, the product of an institution that defied moral boundaries and expectations."

Robert turned out to be a kind of genius as a horse breeder, businessman, and political fixer. He traveled west and made a fortune in the California Gold Rush. At one point, he was "vastly more successful than any of his white brothers." Robert and John were close (Robert's letters to John survive, but not John's to Robert), and Robert eventually made it a personal priority to help his brother get onto the Supreme Court.

Once there, John Marshall Harlan was not always right and not always consistent. But he was able, for the most part, to escape what Canellos calls the "willful obtuseness" of his fellow justices (including that sainted popinjay, Oliver Wendell Holmes Jr.) not just in civil rights cases, but in other cases arising on the cusp of the modern legal era -- including those involving matters of antitrust, income tax, and the incipient American empire.

Plessy v. Ferguson was the 1896 case that upheld the constitutionality of "separate but equal" public facilities for whites and Blacks. As Canellos writes of the doctrine: "In equal parts ridiculous and ingenious, it was designed to provide a legal fig leaf for segregation."

"Our Constitution," Harlan wrote in his fierce dissent of Plessy, "is color blind and neither knows nor tolerates classes among its citizens." It still makes for stirring reading today; no wonder Thurgood Marshall, the first Black Supreme Court justice, called Harlan's dissent his "Bible."