That's the new book by Cass Sunstein and the subtitle is How Like Minds Unite and Divide. I am a fan of Cass Sunstein and I hope they confirm him for OIRA, but I am not persuaded by the main thesis of this book.
I take the main point to be that polarization is increasing and in a bad way. Sunstein offers plenty of good evidence that when people discuss a problem together, they tend to polarize if they disagreed in the first place. But this does not mean polarization is increasing in absolute terms. You can find poll-based public opinion diagrams which point in either direction, but viewing the problem in general and longer-run terms, here are a few offsetting forces:
1. Most of the time people aren't talking with others about controversial problems. During these "cooling off periods" their polarization might well decrease. It's not a one-way ratchet effect.
2. The population is aging in many countries. Even if you believe in the notion of "crochety old men," they are tame crochety old men.
3. Highly polarizing ideologies, such as communism and Nazism, have been on the decline. Maybe jihadism is on the rise but even that is not clear.
4. Wealth and commerce soften morals.
5. Public reactions to the financial crisis have been quite low-key for the most part.
6. Obama goes out of his way to adopt a non-polarizing style (no matter what you think of his policies) and it brings him considerable popularity. That suggests a demand for non-polarization, or at least the perception thereof. In many countries politicians have an incentive to straddle the median and bring outlying groups closer to the center, for purposes of governance and re-election.
Polarization is highly visible in certain segments of the media, including the web. But I am not convinced that increasing polarization is occurring or is a major problem, once we adjust for what one might call "perennial stupidity."