You named some classics already. Lonesome Dove is an incredible book.
Try Charles Portis'
True Grit
Another classic author would be Elmore Leonard. He has some terrific westerns:
Hombre,
Valdez is Coming,
Forty Lashes Less One and several others.
When it comes to Western movies, I favor the Italian spaghetti westerns with their grimier world view and moral ambiguity.
In the late sixties and early seventies, there was a group of English authors who became known as the Piccadilly Cowboys. They wrote books with the same mood as those spaghetti westerns. They have a cult following,so these old books have been resurrected as cheap ebooks.
kyrilson already mentioned George Gilman's
Edge series (which I'll second).
Other good series from these guys would be:
Neil Hunter's
Bodie the Stalker
James Marvin's
Crow
Stand-alone, there's Richard Wyler's
Incident at Butler's Station
Modern authors, I enjoy Peter Brandvold. I'd recommend his five book Colter Farrow series, available as a cheap digital omnibus:
The Saga of Colter Farrow and his standalone:
Blood Mountain
Charles G. West's
Shoot-out at Broken Bow
Marcus Galloway's
Death of a Bad Man