“I am not ghoulish, am I?” Diane Arbus wrote to a lover in 1960, describing how she couldn’t help but stop and watch as a woman lay crying in the street. “Is everyone ghoulish? It wouldn’t have been better to turn away, would it?” For half a century, Arbus’s work has kept us asking these same questions. Her unlikely subjects have become almost proverbial: the twin girls, dressed in...