For most writers, their 100th anniversary looms like a final exam proctored by posterity. A writer who is still being read 100 years after he was born, which usually means at least 50 years after he wrote his major works, will probably keep being read into the future. But for W.H. Auden, who was born 100 years ago today, the century mark feels less like a trial than a celebration. (In fact, it is being celebrated with readings around the country, including one at the 92nd Street Y on March 5.) For when Auden died, in 1973, his immortality was...