Richard Matheson was a writer for the original Twilight Zone, and most of his work shows why. Short and tight, his work has entertained, surprised and amazed whole generations. He became a writer's writer, with many citing him as inspiration and motivation to pursue their own dreams of writing.
A Shadow on the Sun, a short paperback western published in 1994, is not destined to become a classic, but certainly succeeds at being a piece of entertaining fiction, if short. I think that if we were to classify it, it might serve as a good example of the Weird West genre - where the work is a Western with the traditional staples of the genre, laced with elements of Weird Fiction, most commonly a supernatural occurence based on a mythology either borrowed from famous literary predecessors or invented by the author himself.
Matheson's prose style is very sparse, as in most of his other works. He's never the one to use 10 words when two will suffice, always giving the reader just enough detail to not be overtly vague, and leave enough for the imagination to conjure up. This is a great example of this - there's not a word that's unnecessary, and on the novel short lenght quite a lot of characters make an appearance and quite a lot happens.
I've read somewhere that this was originally a screenplay, and it seems it could be. It would do well as an extended episode of the original Twilight Zone, or a TV movie. The suspense is taut, the characters interesting, and you can't just not like a horror novel set in the Old West. It's a short and sweet example of the forgotten paperback novel, where the story was all that counted - it grabbed you by the opening hook and kept interested all the way through to the last page. A good piece of solid writing with an entertaining plot. A very entertaining way to spend an afternoon.