The Op also has a "romance" in one of the Continental Op stories, "The Girl with the Silver Eyes" I think is the name of it. Later, of course, came Spade/O'Shaughnessey; the book version was more like an Op-style "romantic" subplot than the movie.

You may want to try The Glass Key, which was Hammett's favorite. You may also like the Coen brothers movie Miller's Crossing, which is sort of a cross between Red Harvest and Key.