“I’m going to enjoy the time I’ve got left,” Johnson told friends when he left Washington in January, 1969, a worn old man at sixty, consumed by the bitter, often violent, five years of his presidency. He had never doubted that he could have won the 1968 election against Richard Nixon if he had chosen to run for another term. But in 1967 he launched a secret actuarial study on his life expectancy, supplying personal histories of all the males in the recent Johnson line, himself included. “The men in the Johnson family have a history of dying young,” he...