I've enjoyed the film with Humphrey Bogart so I decided to read the original novel. I was surprised at how closely the film hewed to the narrative. Usually a book and a film are only tangentially connected, this was hand in glove.
The only difficulty was with descriptive details. When Hammett starts with "Samuel Spade's jaw was long and bony, his chin a jutting v under the more flexible v of his mouth. His nostril curved back to make another, smaller, v. His yellow-grey eyes were horizontal." I though "geesh!" this isn't the Sam Spade I know.
But that was the only jarring bit of the book. The rest was good. The book's description of the other characters was close enough to those in the film to not noticeably jar as I read.
Too bad Hammett didn't style Spade on Bogart. Oh well.