Charles Kelly was a Salt Lake City printer when he got to know cowboy artist Charlie Russell. The friendship pushed Kelly to further investigate the West, especially its history.
And one of the results: The Outlaw Trail, a self-published 1938 account of the Wild Bunch. Butch Cassidy and the Hole-in-the-Wall Gang had almost been forgotten by that time, but Kelly interviewed a lot of old-timers to get their story (sometimes not accurately).
The book was one of the first to assert that Butch and Sundance died in South America in 1908 (and back it up with evidence).