Hammett is probably best remembered as the author of The Thin Man and The Maltese Falcon, and - - among crime and mystery fiction fans - put down the foundations for what became the modern idea of a pulp/ noir detective. Hammett's creations, Sam Spade (Maltese Falcon) and The Continental Op (short stories, Red Harvest, etc...), would be refined into Lew Archer by Ross McDonald and Philip Marlowe by Chandler. The winding, complex stories would become standard issue for detective fiction, and Hammett's international impact can be felt in places as unlikely as Kurosawa and Leone movies.
In many ways, we're still chasing Hammett.
The leg up Hammett had, aside from an astoundingly punchy and economical prose, was his background as a Pinkerton Detective* and his first hand experience. As well as his time swapping stories with his fellow private eyes.
Hammett himself was as interesting a cat as they come. He left his family, was an ardent leftist and anti-fascist, served in WWI and again in his late 40's in WWII in Alaska - despite a lifetime of health issues, and spent most of his middle-age and to his death as the lover of renowned playwright Lillian Hellman. He served time for his political convictions, and didn't publish any new original fiction for the last 25 of his life.
I've read all of Hammett's novels and a lot of his short fiction. My bookshelf at home is kind of a mess of Hammett and Chandler, somewhat to the neglect of other writers.
I'm maybe a little quick to point to Hammett as the source of everything that came after, but that's okay. I'll be that guy. In my personal pantheon, he's about as important as it gets. And I still very much reading my first Hammett, purchased in a used book store - a 1980's hardback collection of his books, starting with Red Harvest. And it was one of those instances of feeling like you're both entering a whole new world and, also, this is what you've been looking for all along.
Anyway - pick up some Hammett some time. And if not that, put on The Thin Man or Maltese Falcon this week, and have a cocktail for Dash.
*yes, I know the Pinkerton's legacy is complex to say the least