Anything by Upton Sinclair. A lot of people will argue for The Jungle, but King Coal spoke a lot to me once I found myself living in a company-run community for a time. Also, I'd argue the Socialist theme comes out like an after-thought in The Jungle, it's still an excellent novel, though.
Oil! just entered the public domain in the U.S. recently. It was the the basis for the movie There Will Be Blood