Financial desperation and work
William Music has no job to speak of, and though he's traveling back home, there's no job there for him to look forward to. The Great Depression has all but robbed them of hope for work, so that when he arrives at the Kentucky coal mine where he discovers a job, he takes it, zealously. He is desperate, as are the other miners, and though they have jobs, the risk of losing their jobs makes them paranoid and disenfranchised. Music goes from doing his job to walking away when his job involves enforcing Hardcastle's fear tactics; the owner wants them to threaten the union with guns and violence.
Labor rights and unions
The laborers are not all union sympathizers. A lot of them don't feel that much can be done by organizing, but this changes throughout the novel when the workers see how union sympathizers are treated by the boss, and when they decide to organize with the Communists, it is clear that they have been treated inhumanely. Ironically, the need for labor rights and unions arose from Hardcastle's abuse, because he tried to control them by mistreatment and fear tactics.
Life in Kentucky coal mines
To a person who doesn't know anything about Kentucky, this theme is one of the most important in the books. Notice that the main character isn't from Kentucky, but rather, he stumbles into this world on accident. His experience of learn about coal mines is a thematic opportunity for the reader to understand what coal miners experience. They work in small narrow chambers that often collapse, collecting coal from caves that give them black coughs and cancers, and they do this for very little money, because the Depression allows the owner to reduce their pay without fear of competition.
