This is an extraordinary book. The main character, Ned Beaumont, who investigates the mystery is a political fixer for a corrupt politician with a gambling addiction. He is absolutely ruthless and in many ways under motivated. He does some completely horrendous things, including inducing both a murder and a suicide. He is the ultimate antihero. IN Hammett's novels the heroes tend to live by their own code. In this novel, the main character is virtually without a code, and so the novel exists as the limit of the hard boiled detective novel. It is the best work that I have read by Hammett. It has his unique and brilliant sentence composition, but the hero and the situation is taken to this extreme and ends in an essentially nihilistic position. The main character does not care who committed the murder and solves it, essentially because he sees it as part of his job a0s a political fixer. It's clear that he would just as easily cover it up. In this book human nature and human society is seen as red in truth and claw in a way that I have only really seen in Wells' The Time Machine.
It seems to me a first class book, but I can see why it is about Hammett's most obscure, because it is so extreme.