Dashiell Hammett only wrote five novels. Of them, The Glass Key is probably the least remembered. It’s not as iconic as The Maltese Falcon or groundbreaking as the Continental Op novels. And it has none of the screwball comedy tone of The Thin Man. Yet Hammett said this was his favorite, and it’s probably his best.
The Glass Key is about racketeer Ned Beaumont, the loyal henchman to political boss Paul Madvig. Madvig has thrown his lot in with the corrupt Senator Henry, whom Beaumont would like to bring down. However, Beaumont discovers the Senator’s son dead in an alley. Madvig orders Beaumont to cover it up for reasons he doesn’t share beyond wanting to marry the Senator’s daughter.
In the beginning, Beaumont plays the game and does it for personal reasons. He pins it on a bookie who owes him money, tracking the bookie down to New York City. By the time Beaumont gets his money, that lead fizzles. Over time, Beaumont starts looking into the murder himself, at first to protect Madvig. Eventually, he starts to wonder if he’s playing for the wrong team, even fleeing back to New York (where he’s from) at one point.
Beaumont is probably the best drawn of Hammett’s protags. The Op is a violent boy scout. Nick and Nora Charles are thinly veiled cariacatures of Hammett and Lillian Hellman, which does not take away from Red Harvest and The Thin Man. Paired with Hammet’s other morally questionable protag, Sam Spade, Ned Beaumont is the most real of Hammett’s characters. He doesn’t live by a rigid code, but there are lines that should never be crossed. Even the most morally questionable among us, like Spade, who’s a selfish bastard, have lines we won’t cross. No matter how many shades of gray we see things, things do, in fact, reach a point where you have to say, “That’s evil, and I won’t let it pass.”
Madvig somehow crosses that line, and Beaumont is left wondering where his duties lie. Ultimately, they lie with himself, and one can’t really fault him for how he ultimately deals with the the storm around the death of Henry’s son.
And then he says to hell with it.
Edged in Blue 
The Glass Key is the one Hammett novel I’ve not read, which I guess supports your contention that it is a forgotten book. I need to rectify that. What you’ve described sounds like something I’d enjoy.
I’ve seen the movie but never read the book.
This is my favorite Hammett book. It has more of a despairing tone than his other work, a tone that’s more often identified with Chandler.
The Veronica Lake / Alan Ladd movie was pretty good, except the ending didn’t ring true. Another movie, MILLER’S CROSSING, didn’t follow the plot but was mostly inspired by this book, and it was terrific.