what is signicicant about the letter whit reads from the western magazine?

The letter doesn't say much. It demonstrates that the guy who wrote it was literate, and just liked the magazine.

What is interesting is the fact that the guys have a hard time placing exactly who this guy was. I mean, Whit knew him because they had worked quite closely together, but everyone else didn't really get who he was. This shows how independent of each other the guys were. It also illustrates the theme of lonliness and the hard time they had maintaining relationships.

Another item of significance worth note is the fact that Whit seems so impressed that someone he actually knew was in his view famous. This was pretty rare and obviously valued. It shows that people who worked on farms and ranches as migrant workers mattered.