The door has closed forever, said a friend, on a particular part of the past. Or to be more precise, first-person access to the Reagan era through one of its two most important figures has now, with the death of Nancy Reagan, ended. The era itself will never end—it is part of the history of our nation and yielded up its last unambiguously successful president. The spirit of that age: exuberant, expansive. “Where we’re going, we don’t need...