Depressed over the civil-rights movement's direction and burdened by premonitions of his own murder, King told his wife of a mistress in 1968 as Coretta Scott King recovered from surgery for a tumor. "He disclosed to her the one mistress who meant most to him since 1963," writes author Taylor Branch, "a married alumna of Fisk [University], of dignified bearing like Coretta, but different." In his new book, "At Canaan's Edge: America in the King Years, 1965-1968," Branch said the affair had the intensity of a second marriage. Confidante Ralph David Abernathy's wife, Juanita, was furious that "King had picked...