It’s hard to see Elizabeth Varon’s new biography of James Longstreet becoming a runaway bestseller, and that’s a shame, because her study of the Confederate general—one of Robert E. Lee’s closest confidants, yet an outcast in the post–Civil War South for his embrace of Black emancipation and civil rights—is insightful, well-executed, and sorely needed. Part of the problem, perhaps, lies with its dry subtitle. The addition of a single word, though admittedly anachronistic, might have livened things up and drawn in a whole new cohort of readers. Varon describes her famously mercurial subject as “gloomy” and “in a funk,” struggling...