This book has popularized the notion of "social capital." Putnam makes claims that are not justified empirically, but the book is popular, readable and much discussed. It is only slightly related to social networks.
This is a more subtle view of social capital that takes networks into account.
There is no single definition of "social capital" but if there is something of value for organizations, it is important to know this. This paper discusses this and gives a good view of the competing theories and current thinking