On Friday 19 December I finished The Treasure of the Sierra Madre by B Traven, in an old paperback edition which I'd picked up secondhand. The film is a classic - I saw it years ago and remember being really impressed with it - and, I have discovered, much better than the book, which was not a lost classic. Only in the final third did it really hold my attention; the first half in particular was very slow and unengaging, and with a lot of material that I don't remember from the film and suspect wasn't in there. My main memory of the film is the paranoid interplay of the three prospectors, but it takes the book a long time to get there.
(A look at the Wikipedia entries for the book and the film suggest that in fact the film was quite faithful to the book. The film must tell it better.)