ROBERT COLES once wrote, in Farewell to the South: "Maybe I managed to achieve temporary distance on one kind of difficulty by plunging into another." In the context of that book, this sentence refers to Robert Coles's movements back and forth between black and white New Orleans in the 1960's. But it might just as readily be describing the characteristic movement of his mind through the great mass of his knowledge about hundreds of people's lives, and the characteristic way those lives get presented to the readers of his books.