Mark Buchanan's Nexus is a very good introduction to the field of complex networks (one of the most promising fields of complexity sciences).
Written in a quite accessible way, the book shows us how many analogies characterize real worlds networks, including social networks. Such networks generally follow simple rules (the small-world structure, the power law, the preferential attachment, and so forth), that yield extraordinary, "emergent" behaviors.
The knowledge of these laws and structures is crucial in order to understand epidemics, viral marketing, Internet, economic behavior, sexual behaviors, the brain, ecosystems, and many other real world networks.
Buchanan, M. (2002) Nexus: Small Worlds and the Groundbreaking Theory of Networks, W.W. Norton & Company.
For those who read math and want to know more:
Newman, M. (2010) Networks: An Introduction, Oxford University Press.
Barrat, A., Barthélemy, M., Vespignani, A. (2008) Dynamical Processes on Complex Networks, Cambridge Universiy Press.