Of all five of Hammett's novels, "The Glass Key" most resembles a "traditional" whodunit with its linear plot, subtle hints, red herrings, false leads, and disclosure of the murderer in the final chapter. It's his only novel with enough clues to allow readers to figure out who did it--although the identity of the killer will still surprise most readers (including this one, to be honest). What distinguishes it from a typical murder mystery, however, is Hammett's fastidious prose, scurrilous characters, noir ambience, and borderline misanthropy.
Ned Beaumont, a self-described "amateur detective" with an independent streak and a gambling habit, is the loyal underling to shadowy political boss Paul Madvig, whose major concern is to see his candidate, Taylor Henry, reelected to the Senate. When the Senator's son is murdered alongside a dimly lit street, Madvig is the chief suspect, the papers (controlled by the opposition) go on the attack, and Beaumont intervenes with an attempt to clear his boss's name. While not above resorting to ethically dubious behavior, Beaumont retains a vein of rectitude under his tough-guy exterior, and he's even willing to undergo the most brutal thrashings at the hands of the criminal opposition out of loyalty to his own superiors--as long as they themselves don't cross the line.
His fourth novel in three years (1929-1931), "The Glass Key" is bleaker and more cynical than its predecessors, and the mood spirals further downward as the story unfolds. (One can almost imagine Hammett's brooding temper darkening with each stiff drink.) While most of his fiction deals with the underworld and its corruption and squalidness, this work shows most effectively the seedy alliances among businessmen, political bosses, elected officials, law enforcement, media figures, and organized crime in Prohibition-era America.