It's somehow ironic that most of us know this great book mainly because we've seen the also great movie version of the story.
There are a number of departures from the book in the movie, which however terrific is a different experience from reading the text as Dashiell Hammet wrote it. The character of Sam Spade, for example, is presented differently from the now classic Bogie interpretation.
In Hammet's vision of Sam Spade, the private detective presents us with a hero whose crossing of lines, where staying within them violates his own personal code of ethics and his sense of justice, in order to get the job done and the crime solved is more starkly depicted, as is the character himself, who comes off as a more wolfish and in some ways less sympathetic rogue.
The same can be said for the other major characters, including Gutman, Joel Cairo and the beautiful but treacherous femme fatale.
I hasten to add that reading the book and then watching the movie did nothing to detract from either form, instead adding to the attractions of these both.