Reading Bowling Alone was one of those rare life-changing events for me. Putnam described the world I was seeing around me, although frustratingly he stopped short of suggesting any real solutions to the problem of eroding social capital. When I was bleating about this recently, Tris suggested that I should read The Spirit Level. I can't truly say that this book had as much impact om me as Bowling Alone did, but it does address the question of how we could get out of this mess.
It would be easy to criticize The Spirit Level. While it is thorough and well argued, some of the statistical analysis verges on dodgy. This is very much an Economics text rather than a work of Political Science - it is somewhat politically naive, but that's coming from someone who's just finished reading The Prince. These are minor faults which should not put anyone off reading this book.
Read this and this. Then act accordingly.
Update: An anonymous commenter (below) suggests reading The Spirit Level Delusion "Fact-Checking The Left's New Theory of Everything". Comes highly recommended by James Delingpole. Which tells you everything you want to know.