In Chloe Benjamin’s luminous new novel, The Immortalists (Putnam), four siblings, all children, seek out a woman who, they’ve heard, can tell each of them the day they will die. Not how, but when. A casual few words: a month, a day, and a year. They’re given a glimpse around a corner and then go on with their lives, not oblivious exactly, but not unaffected by the information either. The year is 1969. The nation is in turmoil. The deaths of the Kennedys and MLK are behind us. Nixon is president.