Ta-Nahesi Coates, a mild-mannered, even shy writer for The Atlantic, has become a celebrity intellectual as his books about race have become bestsellers. The recipient of a National Book Award and a so-called MacArthur “genius grant,” Coates talks to correspondent Martha Teichner about his latest book, “We Were Eight Years in Power: An American Tragedy”; about white supremacism; and about writing of today’s conflicted America without offering the comfort of happy endings.