In “Slavery by Another Name” Douglas A. Blackmon eviscerates one of our schoolchildren’s most basic assumptions: that slavery in America ended with the Civil War. Mr. Blackmon unearths shocking evidence that the practice persisted well into the 20th century. And he is not simply referring to the virtual bondage of black sharecroppers unable to extricate themselves economically from farming.

He describes free men and women forced into industrial servitude, bound by chains, faced with subhuman living conditions and subject to physical torture. That plight was horrific. But until 1951, it was not outside the law.

All it took was anything remotely resembling a crime. Bastardy, gambling, changing employers without permission, false pretense, “selling cotton after sunset”: these were all grounds for arrest in rural Alabama by 1890. And as Mr. Blackmon explains in describing incident after incident, an arrest could mean a steep fine. If the accused could not pay this debt, he or she might be imprisoned.

Alabama was among the Southern states that profitably leased convicts to private businesses. As the book illustrates, arrest rates and the labor needs of local businesses could conveniently be made to dovetail....