
I've heard rumors that this might be the last book in the series. I'd be sorry if the rumors were true, but if they are, there's thematic support for them. Deadwood is changing, and there are more departures than arrivals in the story, giving it an appropriately elegiac tone.
This whole series is highly recommended and contains some of the author's best writing to my of thinking. You should start with the first book and read them in order if possible. Check 'em out, the sooner, the better.