PRINCETON, NJ -- An Unfinished Life, historian Robert Dallek's new book about the life of President John F. Kennedy, was released this week. The book, among other things, illustrates the gravity of Kennedy's medical problems, describes an alleged affair with a White House intern, and speculates about how Kennedy would have handled the Vietnam War. The publication of this new Kennedy biography raises the question of where Kennedy ranks in the eyes of Americans. Lincoln, Kennedy Have Slight Edge on Greatest U.S. President This Year An April CNN/USA Today/Gallup poll asked Americans: "Who do you regard as the greatest United...