One text related to Of Mice and Men would be Steinbeck's The Grapes of Wrath. It expands many of the ideas in Of Mice and Men. The Joad family is similar to George and Lennie. In both, we see protagonists who have to struggle to realize their dreams. Both works are very similar in their depiction of people who fight against the crushing reality of poverty. The antagonists in both works use money and influence as a way to crowd out others. In addition to this, "sage-like" figures in both works help to provide a moral compass in a world that lacks it. Slim is very similar to Jim Casy. Finally, both works are similar in that they are sad, but ultimately affirm the power of restoration in a world of condemnation. While The Grapes of Wrath is considerably longer than Of Mice and Men, it would be a good complement to it because it underscores many of the same themes.
A different, but related read is Tillie Olsen's "I Stand Here Ironing." This story is about a mother's recollections in raising her daughter during the 1930s. The Great Depression is a shared setting in both Olsen's work and Steinbeck's novella. Both works have very distinct approaches to how poverty impacts emotional connection. One could almost envision a conversation between George and the mother in Olsen's story as they both live in a world where poverty impacts emotions. Both of them had to take care of someone who required so much more than they could give. Olsen's and Steinbeck's works both discuss how economic poverty impacts emotional poverty.
Finally, I'll offer a very different suggestion for the last one. One of the major themes in Of Mice and Men is how dreams influence individual identity. Dreams are a significant part of the story. Everyone has them, from George's and Lennie's shared dream, to Candy joining them, to Crooks, who wishes for a moment to be a part of something. Even Curley's wife yearns for her dreams of being in "pitchers." It might be fascinating to contrast the dreams of these people who are poor with the vision of dreams offered in F. Scott Fitzgerald's The Great Gatsby. While the world of Jay Gatsby is completely different than the "bindle stiff" existence, it is interesting to examine the role that dreams play in both works. In the end, George and Lennie are unable to accomplish their dream. Yet, so is Gatsby. In both settings, the failure of dreams cut across socio-economic lines. It might be powerful to see how broken dreams can be appreciated by both rich and poor. Social class cannot prevent the hurt of our failed dreams.