This noir classic is full of archetypal characters -- the hard-boiled detective, the femme fatale, the loyal Girl Friday, the Fat Man, the sinister, oily foreigner, etc. Not all of these archetypes have aged well, but Hammett's writing is lean and descriptive and tightly plotted, and it helps to remember that this is the book all the cliches are imitating.
Most people have probably seen the movie, and even if you haven't, the "twist" at the end is so old and (by now) such a cliche that it probably won't surprise many readers, but how detective Sam Spade gets there is still deftly executed. Enjoyable if dated, and better than most modern mysteries.