In such popular television series as The West Wing and 24, in thrillers like Tom Clancy's novels, and in recent films, plays, graphic novels, and internet cartoons, America has been led by an amazing variety of chief executives. Some of these are real presidents who have been fictionally reimagined. Others are "might-have-beens" like Philip Roth's President Charles Lindbergh. Many more have never existed except in some storyteller's...