Hardboiled, Terse, Straightforward
If we could only pick one word to describe Hammett's style it would be "hardboiled." And no, we don't mean eggs for breakfast. The term hardboiled is used to describe a kind of prose writing style that is straightforward, witty, and terse. Red Harvest is written in a style that imitates the hardboiled toughness of the Op himself.
For the Op, being hardboiled is a pose, it's a way of looking, seeing, and behaving where you never give away your emotions. It's like having a poker face on at all times, under all circumstances. Hammett portrays the Op's hardboiled toughness through his appearance (trench coat and fedora, of course), his way of speaking (slang and curse words), and his personal habits (drinking and smoking).
Early in the novel, Elihu hits the nail on the head when he describes the Op as a "great talker. A two-fisted, you-be-damned man with [his] words" (5.16). For an in-depth example of Hammett's hardboiled writing style, take a look at the Op's Character page, where we discuss the Op's emotionless reaction to Dinah's death.