In 1918 in Valdosta, Ga., a white planter, Hampton Smith, was killed by a rifle shot into his home. His wife, wounded in the shooting, accused a black man, Sidney Johnson, of doing it. But he could not be found. So a frustrated lynch mob hanged other black men instead, including Haynes Turner.

Mary Turner, his wife, eight months pregnant, protested his murder and swore to seek justice. She, too, was seized by a mob. She was stripped, doused with gasoline, hung by her ankles from a tree, then set afire. As she burned, a man cut open her belly, dropping her baby to the ground. It let out a cry, and he stomped it to death.

This took place before a crowd in the hundreds, including many children.

Walter White, an investigator for the National Association for the Advancement of Colored People whose skin was so light he could pass for white, got the details from numerous witnesses and sent the names of 17 people involved to the governor, who did nothing.

This is an account you’re not likely to find in American history textbooks, most of which barely cover the reign of terror against the black people of the South (and near-South) from the 1890s into the 1940s (with sporadic incidents after that).

At the peak of these horrors, white supremacists murdered hundreds every year, many in so-called “spectacle lynchings,” with huge crowds watching, eating and drinking and enjoying themselves as black people were tortured, then hanged, shot or burned alive. Photographers took pictures for souvenir postcards. Other souvenir hunters took parts of the victim — ears, fingers, penis — often before killing him.

You would think that decades of racially motivated murder would be impossible to overlook in any account of American history after the Civil War. But you would be wrong. Many history textbooks don’t even acknowledge the deep racism of the period, much less the catastrophic consequences.

That’s why “At the Hands of Persons Unknown” is such an important book. It records in exhaustive, revolting detail the horrors perpetrated by white racists and the courageous efforts by black writers and activists including White, Ida B. Wells-Barnett, W.E.B. Du Bois and James Weldon Johnson to mount an anti-lynching movement. (They had better luck raising an outcry in Britain than at home.)

To read this book is to reel from the horror, to be sickened and ashamed, and to comprehend the buried reality that America once was the most viciously racist country on Earth.