In September, a New York Times review of a new biography of Adolf Hitler was shared so widely that the 1,000-page book shot up Amazon’s bestseller list. But many people weren’t sharing the article because of what they’d learned about Hitler. The chatter was about the man never mentioned, lurking just between the lines.

The Times book critic, Michiko Kakutani, listed several factors that had helped Hitler rise from a ridiculed rabble-rouser to a nightmarish dictator. She noted that the German leader was an eccentric but compelling speaker who whipped up massive crowds at theatrical rallies, that he was endlessly untruthful and compounded his lies via the latest technologies, that he was an egomaniac who trumpeted slogans about how he alone could make Germany great again and restore law and order, that he exploited economic troubles and popular frustration with political gridlock, that he viewed the world in Darwinian terms and had a thoroughly dark vision of the state of his country, that his opponents and reluctant allies repeatedly downplayed the danger of his demagoguery.

Kakutani described the biography—written by the German historian and journalist Volker Ullrich and focused on Hitler’s life through 1939—as a “parable.” The parable’s lesson, readers quickly concluded, was of the untold hazards of electing Donald Trump president of the United States. It wasn’t the first time during the presidential election that people had compared Trump to Hitler, and it wouldn’t be the last. But Kakutani’s apparent warning, in letting history speak for itself and leaving the rest unsaid, was the most bone-chilling.

History, however, is best read within the lines, not between them. Ullrich’s biography has hundreds of pages of lines, and many of those lines don’t bring Trump to mind. Imagine if, for example, Kakutani had also highlighted Hitler’s military service in a world war, his limited romantic involvement with women, his participation in a violent coup and resulting imprisonment in the 1920s, or the virulently anti-Semitic views he expressed before coming to power in Germany. Readers might not have grasped the implicit message of Kakutani’s article (assuming Kakutani intended her review as an allusion to Trump, which she has neither confirmed nor denied). The signal would have been scrambled.

Ullrich, for his part, had to read Kakutani’s review twice before concluding that it “was intended to do something more than just praise my book.” He doesn’t agree with the subtext that many people spotted. ...