Bowling alone is a great read on social capital from author Robert Putnam. He emphasizes the importance of trust and reciprocity, and shows the tangible benefits that result from strong social ties.
From Putnam:
What does “social capital” mean?
The central premise of social capital is that social networks have value. Social capital refers to the collective value of all “social networks” [who people know] and the inclinations that arise from these networks to do things for each other ["norms of reciprocity"].