That book related his search among contemporary historians for an adequate explanation for Hitler's evil—and the underlying tension between "exceptionalist" explanations (Hitler was off the grid of 'normal' human behavior) versus naturalist explanations (Hitler was at the end of, but within, a continuum of human behaviors).
That book examined his search among modern historians for an adequate explanation of Hitler's wickedness—and the opposing ideas between "exceptionalist" theories (Hitler was outside 'normal' human behavior) and "naturalist" theories (Hitler was at the end of, but part of, a spectrum of behaviors).