"The Natural" is a quiet, contemplative novel that uses the mythology of baseball to frame a poetic parable of fate...of the idea of "what could have been."
Baseball, more than any other sport, has a history composed equally of fact and legend. That's its' charm. Using that gauzy place between the real and the myth, Malamud tells the tale of Roy Hobbs, the greatest baseball player who ever lived, but who hardly ever played.
Hobbs' life, at least the part we are privy to, is shaped by his decisions and actions surrounding three women. They each, and I'm reducing this to absurdity, represent a basic ideal: home-spun decency, harsh reality and seductive temptation. It could be said that where he ends up at the end of the novel is determined solely by the choices he makes regarding each woman. His character becomes better defined as the reader discovers Hobbs' feelings towards each as well.
It's difficult not to see Robert Redford's face in the mind's eye, nor to hear Randy Newman's music in the background whilst reading the book. Those images and sounds have penetrated popular culture so deeply, it doesn't matter if you haven't seen the movie.
Read the book first, then see the movie. They actually make the other better.