Of how many magazine articles can you recall where you were and what you felt when you read them? If any occur, there’s a reliable chance John Hersey’s ‘Hiroshima’ will figure among them, and not just because it will have been assigned at an impressionable age in school. For the first time in its history, the New Yorker cleared an entire issue in August 1946 to run the 30,000-word piece in full.