If you're interested in graphs, network science, or social science, Barabási's Linked is a rewarding read. (https://www.amazon.com/Linked-Everything-Connected-Business-...)

There's also stuff under "Network theory"[1] at Wikipedia. I feel like those two articles should probably be merged, but it hasn't happened yet, and I haven't had time to take a stab at it. But anyway, both articles contain some useful info.

I also recommend these few books as a good starting point:

Network Science: Theory and Applications[2]

Linked: How Everything Is Connected to Everything Else and What It Means[3]

Six Degrees: The Science of a Connected Age[4]

The Wisdom of Crowds[5]

Nexus: Small Worlds and the Groundbreaking Science of Networks[6]

Diffusion of Innovations[7]

Of course - being that Network Science is a multidisciplinary field, that touches a lot of other areas - it can be hard to get a handle on what to study. But those few books - between them - cover a lot of the basics and would give somebody who's interested in this stuff enough background to figure out where to start digging deeper.

For a little bit more on the technical side, a couple of good resources at:

Introductory Graph Theory[8]

Introduction to Graph Theory[9]

Algorithms in Java: Part 5 - Graph Algorithms[10]

[1]: http://en.wikipedia.org/wiki/Network_theory

[2]: http://www.amazon.com/Network-Science-Applications-Ted-Lewis...

[3]: http://www.amazon.com/Linked-Everything-Connected-Else-Means...

[4]: http://www.amazon.com/Six-Degrees-Science-Connected-Edition/...

[5]: http://www.amazon.com/The-Wisdom-Crowds-James-Surowiecki/dp/...

[6]: http://www.amazon.com/Nexus-Worlds-Groundbreaking-Science-Ne...

[7]: http://www.amazon.com/Diffusion-Innovations-5th-Everett-Roge...

[8]: http://www.amazon.com/Introductory-Graph-Theory-Gary-Chartra...

[9]: http://www.amazon.com/Introduction-Graph-Theory-Dover-Mathem...

[10]: http://www.amazon.com/Algorithms-Java-Part-Graph-Pt-5/dp/020...

http://www.amazon.com/Linked-Everything-Connected-Else-Means...

He was just joining up with the CS dept at ND when I was doing my msc there and I got to see him speak a few times. This is part of a general trend, we naturally appear to keep small social circles on average with a few "hubs" providing most of the connections to others. This is actually a pattern in distributions throughout the natural world, it's really an interesting topic to study.