One night in the autumn of 1944, two Frenchwomen—Loulou Le Porz, a doctor, and Violette Lecoq, a nurse—watched as a truck drove in through the main gates of Ravensbrück, the Nazi concentration camp for women. “There was a lorry,” Le Porz recalled, “that suddenly arrives and it turns around and reverses towards us. And it lifts up and it tips out a whole pile of corpses.” These were the bodies of Ravensbrück inmates who had died doing slave labor in the many satellite camps, and they were now being returned for cremation. Talking, decades later, to the historian and journalist Sarah Helm, whose new book, “Ravensbrück: Life and Death in Hitler’s Concentration Camp for Women” (Doubleday), recounts the stories of dozens of the camp’s inmates, Le Porz says that her reaction was simple disbelief. The sight of a truck full of dead bodies was so outrageous, so out of scale with ordinary experience, that “if we recount that one day, we said to each other, nobody would believe us.” The only way to make the scene credible would be to record it: “If one day someone makes a film they must film this scene. This night. This moment.”

Le Porz’s remark was prophetic. The true extent of Nazi barbarity became known to the world in part through the documentary films made by Allied forces after the liberation of other German camps. There have been many atrocities committed before and since, yet to this day, thanks to those images, the Nazi concentration camp stands as the ultimate symbol of evil. The very names of the camps—Dachau, Bergen-Belsen, Buchenwald, Auschwitz—have the sound of a malevolent incantation. They have ceased to be ordinary place names—Buchenwald, after all, means simply “beech wood”—and become portals to a terrible other dimension.

To write the history of such an institution, as Nikolaus Wachsmann sets out to do in another new book, “KL: A History of the Nazi Concentration Camps” (Farrar, Straus & Giroux), might seem impossible, like writing the history of Hell. And, certainly, both his book and Helm’s are full of the kind of details that ordinarily appear only in Dantesque visions. Helm devotes a chapter to Ravensbrück’s Kinderzimmer, or “children’s room,” where inmates who came to the camp pregnant were forced to abandon their babies; the newborns were left to die of starvation or be eaten alive by rats. Wachsmann quotes a prisoner at Dachau who saw a transport of men afflicted by dysentery arrive at the camp: “We saw dozens . . . with excrement running out of their trousers. Their hands, too, were full of excrement and they screamed and rubbed their dirty hands across their faces.”

These sights, like the truck full of bodies, are not beyond belief—we know that they were true—but they are, in some sense, beyond imagination. It is very hard, maybe impossible, to imagine being one of those men, still less one of those infants. And such sights raise the question of why, exactly, we read about the camps. If it is merely to revel in the grotesque, then learning about this evil is itself a species of evil, a further exploitation of the dead. If it is to exercise sympathy or pay a debt to memory, then it quickly becomes clear that the exercise is hopeless, the debt overwhelming: there is no way to feel as much, remember as much, imagine as much as the dead justly demand. What remains as a justification is the future: the determination never again to allow something like the Nazi camps to exist.

And for that purpose it is necessary not to think of the camps simply as a hellscape. Reading Wachsmann’s deeply researched, groundbreaking history of the entire camp system makes clear that Dachau and Buchenwald were the products of institutional and ideological forces that we can understand, perhaps all too well. Indeed, it’s possible to think of the camps as what happens when you cross three disciplinary institutions that all societies possess—the prison, the army, and the factory. Over the several phases of their existence, the Nazi camps took on the aspects of all of these, so that prisoners were treated simultaneously as inmates to be corrected, enemies to be combatted, and workers to be exploited. When these forms of dehumanization were combined, and amplified to the maximum by ideology and war, the result was the Konzentrationlager, or K.L. ...