Brave New World, Aldous Huxley. (Any used or inexpensive copy is fine)
The Righteous Mind, Why Good People Are Divided by Politics and Religion, Jonathan Haidt. (Any used copy is fine)
Asking the Right Questions: A Guide to Critical Thinking (10th Edition), M. Neil Browne and Stuart M. Keeley.
You must buy the tenth edition of Asking the Right Questions, because there are readings in this edition that are not in earlier editions. You can purchase used copies from Amazon.com.