There was never an emergency when Hahn was at the wheel. (She was beautiful, which never hurts, and came from a well-to-do family of German Jews in St. Louis.) Her writing made great use of offhandedness. She was on her way to Congo in 1935 “to forget that my heart was broken; it was the proper thing to do in the circumstances.” In a “Letter from Brazil” from 1960, she casually mentions that her host “woke up one morning to find his pajamas spotted with blood; he had been bitten by a vampire bat.” She roamed the world, seemingly without fetter. “It had become clear to me on the first day in China that I was going to stay forever, so I had plenty of time,” she writes in “The Big Smoke.”