This book reads almost verbatim to the movie and I loved it! There are a couple of differences which I noticed and found interesting. The first is that Sam Spade has an affair with Brigid O’Shaughnessy . The second is that Joel Cairo and the young thug Wilmer are apparently lovers as well. Given the era during which the movie was produced, it’s not surprising that these facts were kept from the theater-going public.
If you haven’t read the book or seen the well-known movie, I highly recommend both. They are classics.