Garton, L., Haythornthwaite, C., & Wellman, B. (2006). Studying Online Social Networks. Journal of Computer-Mediated Communication, 3(1). doi:10.1111/j.1083-6101.1997.tb00062.x
This paper gives a great introduction to SNA, covering basic concepts, units of analysis, data collection, and data analysis. Although everything is not in great detail, but it’s a great place to start unless you wish to start with a huge textbook.