The book starts out strong with economic and environmentalist principles and how people abuse these metrics to push an agenda. The book also contains a really harsh look at what Marxism and communism did in the last century (~100M dead and the trashing of several countries' economies, educational systems, and civil liberties) that fires a shot across the bow of anyone who suggests that communists "meant well."
The middle of the book is filled with conjectures surrounding the psychological state of the hateful left. I'm aware that Phil DeMuth has a degree in psychology, but this seemed like a bunch of name calling, despite the fact that the research they criticize (with funding from the National Science Foundation, no less) is far worse. It makes me question how categorizing conservatism as a mental illness passed the peer review process of scientific journal publication.
In short, the authors suggest that throughout United States history, we have generally progressed towards the right thing. We have done stupid and immoral things along the way, but we are slowly progressing towards racial and gender equality and mutual prosperity. Given the global alternatives, the United States is a great place to live. For example, people should realize that if we're arguing over whether or not DVDs can be legally played under Linux in our country, then we have obviously solved the issues about feeding our people (a majority of Americans are obese), protecting them from external enemies (9/11 notwithstanding), providing gainful employment and universal education, and other fundamental needs that billions of the human race will never enjoy. We may not agree with our politicians' views and are encouraged to actively oppose their policies, but we should never deny the legitimacy of our country and how great a place the United States has become.