As I said before, I saw the movie long before I read the novel, so when the various characters in the book were conversing, I was hearing their conversations in the voices of the actors. This was made even easier by the fact that whole swathes of the film dialogue are lifted almost unchanged from the book. The movie actually follows the book very closely, only with more profanity and we're told straight out that Spade and Brigid have a sexual relationship. This of course was impossible to show in the film due to the previously mentioned Hays Code. The existence of this relationship in the novel, however, makes Sam's statements to Brigid when he turns her in more believable. In the film, his hesitation to hand her over to the cops seems a little unlikely, as does his statement that he might love her; nothing which has been shown justifies these things.
The female character whose role is more substantial in the novel than in the film is that of Effie Perine, Spade's secretary. Effie is the only woman in Sam's life whom he trusts and respects. Which is why she is safe from his womanizing ways. She is always ready to do what is necessary to help Sam out of the trouble he's gotten into, even if she doesn't approve of how or why he got into it. I generally like her character in the book, as she is loyal and sensible... until the end of the novel, when she annoys me. After Sam has handed over Brigid to the police, he returns to his office where Effie is waiting for him. Weirdly, she is upset with Spade for having turned O'Shaughnessy in, which seems totally out of character. Effie only actually met Brigid a couple of times, and then only to show her into Sam's office; she barely knew her. And Brigid betrayed her boss, whom she's very fond of, over and over. Effie should have been enthusiastically waving as the paddy wagon carted Brigid off to chokey, but instead she is angry, and thinks that Sam betrayed the murderess. It makes no sense.
So those are my thoughts on The Maltese Falcon. It doesn't, in my opinion, have a really great mystery plot... we're never really in doubt who's responsible for the crimes, and the novel takes a lot of unnecessary twists and turns. But it is a interesting study of fascinating and deeply flawed characters.