Magazine science fiction of the 1950’s placed a premium on clever premises. Only rarely were those premises worked out in plausible stories about believable people in authentic-seeming futures. Gunn’s characters in “The Immortals” were clearly created to serve the needs of the plot. The projected future — an America whose social fabric has all but disintegrated — feels thin and arbitrary; it is hard to imagine anything going on in this world outside the narrow spotlight of the narrative. Readers with fond memories of yesterday’s science fiction will enjoy the deja vu quality of “The Immortals.” Others might use as a yardstick to see how far the genre has come in wedding mind-opening premises to the time-honored virtues of literate fiction.