My favorite author for westerns has always been Louis L'Amour. The characters in his books seem alive and the way he describes the land is wonderful. I would not describe his westerns as dark though.
I bought The Desert Crucible by Zane Grey from Amazon yesterday. This is the sequel to Riders of the Purple Sage. The original publishers released an abbreviated and heavily censored version. This is the original as Zane Grey wrote it and I am looking forward to reading it.
http://www.amazon.com/gp/product/B00...?ie=UTF8&psc=1
Apache