I finished The Children of Men by PD James last week.
The book and movie very different. About the only things shared between the book and movie are characters and of course, the premise of a woman suddenly getting pregnant.
I don't think that subtracts from either one, though. They're both excellent, and quite honestly, if they made a faithful movie from this book, it would have been boring. The immigration issue was present in the book, but expanded and brought more up-to-date in the movie. (The book was published in 1992.)
I'm still chewing it over, but it was more of a study of one man's thought process in a future that really had no future, and then suddenly waking up from his malaise.