Someone pointed me to a BBC documentary Six Degrees of Separation which discusses network theory. This is very insightful. Here is the video in its parts. The total running time is about 1 hour and worth watching. (I got this from this URL which someone sent me.)
Several interesting links were referred to in this documentary and which I stumbled across as a result of watching this documentary:
My first question when I saw this was the difference between network theory and graph theory. Network theory seem to try and explain that many things exist as networks. Graph theory seems to be an approach to explain a network. I could be wrong.