I'm currently reading John Wayne: The Life and Legend, by Scott Eyman. I have found it to be a very enjoyable read that focuses on his career as an actor, producer and director. Some stuff about his personal life is thrown in to give context. Anyway, it covers some interesting background behind pretty much each of his movies,
One of his quotes from 1957 cited in the book: "That guy you see on the screen isn't really me. I'm Duke Morrison, and I never was and never will be a film personality like John Wayne. I know him well. I'm one of his closest students. I have to be. I make a living out of him."