I've expressed my liking for the earlier books in this series here and here. And probably everybody knows that Mike Jameson is really this guy. This time out he takes on one of the most famous episodes of the Old West, the killing of Wild Bill Hickok, and he does a terrific job of it. You could just read the book for this bit of writing, which is the centerpiece, but there's plenty more, as some of the plot threads from previous books come together in ways that might surprise you.
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.