Nexus: Small Worlds and the Groundbreaking Theory of Networks
You've probably heard that no-one is more than six handshakes away fromt the President of the USA, due to what is known as the small-world effect. In Small Worlds Mark Buchanan illustrates this effect with plenty of examples. In an easy to read book he shows how the connections in typical networks makes such an effect pretty much inevitable, and he goes on to look at some of the consequences of this effect - how systems often have a 'tipping point'. If you want to find out about the small world effect, I would say that this book is an excellent place to start.
The book doesn't just deal with social networks, but with many examples of the small world effect. There are chapters on the workings of the brain, the internet, and the spread of disease. Buchanan also looks at the connections of ecological systems and the role of networks in economics - why the rich get richer. Indeed my one complaint about the book is that it tries to relate too much to the small world effect. By being so wide ranging, it seems to move away from a theory with specific predictions towards one which can explain anything (and so may well end up explaining nothing)