“Last of the Breed” by Louis L’Amour
“Lucifer’s Hammer” by Larry Niven and Jerry Pournelle.
I LOVED “Last of the Breed”!!! Highly recommended!
I never got into the whole Cowboy theme with Louis L’Amour, however, he did have one novel I loved as a kid and read it over and over. I forgot the name but it was the one with the Air Force pilot who was shot down over Siberia and had to walk across Russia avoiding their military and surviving the -50 winters.