“Guerrillas do not keep records,” writes Edward Ball, author of “Life of a Klansman: A Family History in White Supremacy.” “They operate at night and in secrecy.” Nonetheless, through exquisite research and with the help of a file maintained by a schoolteacher aunt, Ball has managed to re-create the life and times of Polycarp Constant Lecorgne (1832-86), a New Orleans ship’s carpenter and the author’s great-great-grandfather, who was a Confederate soldier and a devoted white militant supremacist during Reconstruction.

Lecorgne was a family hero. He “fought for whiteness, for our tribe,” says the author. “I’ve known about him since childhood. I have been afraid of his story.”

Ball has confronted his family’s past before, in his National Book Award-winning “Slaves in the Family” (1998). Here he meticulously describes a Confederate ancestor’s role in helping to re-establish white supremacy in Louisiana after slavery’s abolition. A foot soldier in white militias affiliated with the Ku Klux Klan, Lecorgne rode with nighttime gangs that terrorized African Americans; took part in an 1866 riot in which dozens of Blacks died, and boasted of a head wound suffered eight years later at the Battle of Canal Street, a major guerrilla insurrection against the Reconstruction-era Louisiana government.

Perhaps most disturbing, Lecorgne was a “bland and ordinary” white man of his time. An indifferent Confederate soldier and an angry member of the working class who longed for his family’s once higher position in New Orleans’ racial pecking order, his life as a white supremacist was both “insignificant and yet calamitous,” writes Ball.

The author notes that some 50% of today’s white families are descendants of such Ku Klux Klan members. Their ancestors helped make white supremacy a “bedrock” of American life. Ball insists readers consider that legacy’s continuing impact.

Ball places his ancestor’s story against the complex 19th-century racial mix of New Orleans and the national upheaval of Reconstruction. He details the pseudoscientific arguments in favor of white supremacy offered by Samuel Morton, Louis Agassiz and others in magazines like De Bow’s Review. And he covers minstrelsy and other behaviors that help cement racial attitudes.

There is much we do not know about Ball’s great-great-grandfather. Did he disguise himself in a robe and mask? How many atrocities did he engage in? The answers are unrecorded. Ball sifts through the uncertainties, fills in gaps using inference and implication, and successfully renders a disturbing story of a Klansman.

Lecorgne’s life in white supremacy is “both exceptional and normal,” writes Ball. “It is aberrant in its cruel extreme. And yet it is typical, because the wider community tacitly supports it.”

In this turbulent era, he observes, “whiteness enters the social unconscious of the country.”

 Joseph Barbato, an author and journalist, is a former contributing editor at Publishers Weekly. He has reviewed for the Washington Post and the Los Angeles Times.

 Life of a Klansman

By: Edward Ball.

Publisher: Farrar, Straus & Giroux, 416 pages, $28.