W. D. Howells once wrote to his friend Mark Twain of "the black heart's-truth, which we all know of ourselves in our hearts" -- the dark core of inner life that underlies "the whity-brown truth of the pericardium, or the nice, whitened truth of the shirt front." For Howells, a writer with a lifelong history of psychological disturbances, telling this "black heart's-truth" evinced his courage and imaginative spirit. John W. Crowley examines psychological...