Check out this article by Caryn James from The New York Times which discusses both the PD James novel Children of Men and the film.
Here’s a taste –
No one should have to choose between Clive Owen and P. D. James. As an alcoholic, unshaven hero in a totalitarian near-future, Mr. Owen holds together the ominous yet vibrant new film “Children of Men,” adding to his list of brooding, darkly handsome characters (notably in “Closer”). But while this Alfonso Cuarón film is inspired by the 1992 James novel, the movie is so purely cinematic, and its plot departs so widely from the book’s, that the screen version may obscure how wonderfully rich and unlikely that novel is.