The Maltese Falcon is a very good movie. It is beautifully directed, and the cast is perfect. But there are subtleties in Hammett's writing that are missing in that movie. Those subtleties, of course, are what makes the book so much better than the movie.
Hammett has an ear for characters -- all of his gangster-types sound very different, but in a way that is consistent with what we are told about them. That ear generally marks an excellent writer.