Who wrote the letter that is published in pulp magezine of mice and men?

There's no letter published in "Of Mice and Men" that we know of.

"Of Mice and Men" is a novella by John Steinbeck, and it doesn't feature any letters in its narrative. Perhaps you're thinking of a different work, or there may be a misunderstanding about the content of the novella.

Could you please clarify what you're looking for? Perhaps you're thinking of a different book, or maybe you're referring to a letter written by Steinbeck about the novella.