I ended up finishing the book, after all. And it turned out that it was a pretty good one, all in all. While I still stand by what I said in the first review, the action picks up after that point.
The characters are good, and Sam Spade the detective and protagonist, is 'quite a character' as the main villain, Gutman says. He is in control in a masterful, cool way, and must have been brought out to good effect by Humphrey Bogart in the film version. I admire the way in which the plot is revealed, and the twists and turns as it is revealed.