Oh, well. At least I mentioned "Lin McLean" first.
I guess everyone has read "A Texas Cowboy, or Fifteen Years on the Hurricane Deck of a Spanish Pony," published by Charles A. Siringo in 1885. Partly "autobiographical," it has stories about events that he had heard about, and incorporated into his own life. As such, it can be considered the first cowboy western.
Written when he was not quite 30, he calls himself "an old stove up cow puncher who has spent nearly a lifetime on the great western cattle ranges."
A sketch of Siringo in the first edition of 1885 shows him leaning on a stump, in woolly chaps, a Bowie knife on his belt and a rifle in hand, wearing suit coat, vest and tie, and an Indian tipi in the background. He is shown in a frontispiece, mounted, with Bowie and pistol on a cartridge belt, similarly attired. Another frontispiece illustration shows him in the foreground, mounted, with the chuckwagon and camp cook, two punchers roping a longhorn, and a group in the background moving a herd.
In spite of not being wholly autobiographical, the story is true to life. The best edition for not too much money is the attractive 1950 production by William Sloane Assoc., NY, with illustrations by Tom Lea, an introduction by J. Frank Dobie, and facsimiles of the frontispieces from the first edition.

The Dobie Introduction talks about other early western writers, including Eugene Manlove Rhodes, mentioned above. "While riding on a horse-buying expedition, he was arrested by vigilantes for a horsethief and taken to a cow camp. .... "A well-educated man among the vigilantes took the lead in cross-examining him.
"Where were you in 1881?," he asked.
"In Canada."
"Where were you in 1880?"
"In Montana."
"Well, where were you in 1879?"
"In Oxford University."
Undoubtedly the dialogue is partly invented, but the story is true.