Wesley S --The Cowboy and the Cossack is my all-time favorite Western book. I wore out my first paperback copy and had to use an out-of-print search and buy hardback to replace it. I've read that thing at least once a year since I discovered it, and don't think I'll ever get tired of it.
Didn't know it was based on a true story, though--where did you find out about that?
Lin