If you're interested in this, I highly recommend the book Networks, Crowds, and Markets [1]. It's an awesome (undergrad-level) introduction to graph theory, and has a ton of practical information on how it applies to modelling networks of people, as well as economic things like auctions and (game-theory) games. Since it's a textbook, it also has full exercises and advanced materials.

[1] http://www.amazon.com/Networks-Crowds-Markets-Reasoning-Conn...