Ethnicity is another of those "contested" concepts that causes fights and posturing. This week, we'll read the most-cited (and still often-cited) essay in social anthropology on the topic. (Apologies for the marked-up scan.)
Fredrik Barth. 1969. Introduction. In Ethnic Groups and Boundaries, F. Barth ed.
Second, to get a broader idea of approaches to ethnic and other kinds of social groupings, here's a recent Science paper that describes a theoretically-motivated set of experiments on how norms and ethnic-like markers can co-evolve. One of the motivating models is my own, so I have some stake in this stuff, but I am by no means convinced that this is an important process in real societies.
Charles Efferson, Rafael Lalive, and Ernst Fehr. 2008. The coevolution of cultural groups and ingroup favoritism. Science 321:1844.