Hammett was too good an artist and too much of a professional by now to write the same book more than once. In "The Maltese Falcon," he'd more or less invented the private-eye novel -- and exhausted its hero's moral options by the final scene. In "The Glass Key," he'd produced an existential fable, years before Sartre's or Camus's. By then, there was nothing left for him but comedy (albeit a comedy with fangs). And after "The Thin Man," there really was nowhere to go for a writer born in 1894 who had served his country in World War I, become disabled with tuberculosis and would serve again in World War II. "If you are tired you ought to rest, I think," Hammett wrote in a fragment of one last attempted novel, "and not try to fool yourself and your customers with colored bubbles."