There is great effervescence out there as people talk about how internet is revolutionizing the way people interact, consume information, create content, share information and collaborate. Whether this revolution is good or bad, society is definitely never going to retreat to the old ways of a pre-internet world.
We're left to deal with these changes, and this book does a great job in explaining the source and fundamentals of this revolution, going back in history a far as Johannes Gutenberg in the 15th century to explain the societal revolution caused by the invention movable type printing and how the Internet has caused a similar revolution, yet more profound, by bringing the costs of publishing and sharing knowledge down to marginally zero.
This is where the term "Cognitive Surplus" comes from. Sharing knowledge have become so ridiculously inexpensive that this surplus is invading the internet, leveraging amazing feats like Wikipedia that could have never been leveraged before.
Outstanding work, helps you understand the profound changes this revolution still have on the works and how to benefit from it. One critic, though: the book goes on for almost its entirety laying the foundation for a somewhat convoluted and short conclusion (final chapter, about 30 pages) at the end. I wish the conclusion of the book could be as well elaborated as its other chapters. Having said that, I still think it is great book and definitely worth reading.