Like the equally excellent "Here Comes Everybody" (HCE) by the same author, this is about social media. In HCE he concentrated on how social media is empowering people in ways that are both challenging to corporate and government power, and liberating the forward thinking companies who are ready and able to embrace this.
Cognitive Surplus picks up from where HCE left off, and discusses how we can use the power to "do good" that has been released due to have more leisure time in modern culture. Just like social media has empowered people to challenge centralised authority (HCE) so it has freed us to be more effective in our altruism. The sub title says it all "creativity and generosity in a connected age".
The book surveys the enabling factors (means, motive and opportunity), showing how social media powerfully adds in each of these areas, and also looks at culture, community and individualism (and the tensions therein). Finally Shirky deals with how this has come to pervade modern life and culture and how it's changing us.
Perhaps one omission is that he doesn't deal very much with the negative side - social media can be an equally pwerful tool in the hands of those less altruistic: cyber-bullying, paedophilia, etc., being at the darker end of the spectrum.
As ever Shirky is always readable, well researched and challenging. This is a "must read" for anyone interested in social media.