Carlin Romano, critic at large for The Chronicle, is a professor of philosophy and humanities at Ursinus College and author of America the Philosophical (Alfred A. Knopf, 2012).

Adolf Hitler loved books—that nasty bent for book burning notwithstanding—and the book industry loves him back. Type his name into Amazon, and while he doesn't trigger the English-language numbers of Jesus (186,740) or Lincoln (70,710), he registers a solid 18,597—a stunning figure for someone who died less than 70 years ago.

There's no reason to react only cynically, mumbling that sensationalism and extreme violence sell books, and so what. In light of Hitler's heinous impact on the world as paradigm of evil, despoiler of countries, mass murderer of Jews, non-Jews, the disabled, fellow Nazis, and pretty much anyone else who got in his way, the attention to every angle of his life makes sense. As the U.S. Holocaust Memorial Museum recently showed in its startling report that the Nazis ran 42,500 camps in Europe rather than the previously estimated 7,000 or so, believing that we already know everything we need to know about the Holocaust is profoundly premature. Let historians focus on Hitler's military strategy one year, on his library the next. Ultimately we move toward a fuller picture of what every affected party might call "the catastrophe."

Yet one distinction traditional to many philosophers, and still fundamental in common-sense culture, rarely gets central discussion in the avalanche of material about Hitler: reason versus emotion. The questions run in two directions.

Was Hitler rational or a hyperemotional madman? Did he make reasoned decisions on the whole, even philosophical ones, however morally abhorrent they were?

And how do we, or should we, react to him long after his suicide perhaps cost the world the most provocative of "Hitler books"—his postwar memoir? Is cold, rational assessment ultimately the only respectable reaction? Or is that itself an outrage? Must his immense moral crimes trigger not just somber condemnation in us but also ferocious anger, bristling moral fervor, barely stoppable tears?

Three new books provide an opportunity for mulling over those questions without zeroing in on them directly. Hitler's Philosophers (Yale University Press, May), by Cambridge-trained historian Yvonne Sherratt, examines Hitler's enthusiasm for philosophy, the thinkers in the field who prefigured and fueled his ideological leanings, as well as (awkwardly, given her title) some prominent figures who opposed him. Hitler's Charisma: Leading Millions Into the Abyss (Pantheon, April), by Laurence Rees, longtime director of the BBC's history programs and author previously of books including Auschwitz: A New History, sedulously takes us through Hitler's career with regular attention to how his much-attested-to charisma, which worked on many but not on all, empowered him. Otto Dov Kulka's Landscapes of the Metropolis of Death (Harvard University Press, March) presents the most intractable document of these three—a noted Israeli historian of the Holocaust, committed in his career to sober, scholarly work about it, shares his memories and dreams of the time he spent in Auschwitz at the ages of 10 and 11....