Environment or "social responsibility" category?
Granted, nearly all books that had anything to do with the environment was published after the 50s, so it probably wouldn't be in the public domain, but I'd love to see books by Rachel Carson and Françoise d'Eaubonne.
I'd also love to see authors like Naomi Klein, Eve Ensler, or Kelsey Timmerman. All contemporary, but I figure the authors would be more willing to spread the word? Maybe?