Whiteness of a Different Color by Matthew Fry Jacobson
It's a standard thing in all of my graduate seminars to discuss the ways in which race (and gender) are social constructions. Jacobson tries to understand how European ethnics went from being Celts, Slavs, Italians, etc. to being white in the eventual racial bifurcation of the country along a black/white axis. It was difficult as a reader in modern America to reorient the way I think about race/ethnicity in order to understand Jacobson's argument. The book is mostly effective, but the middle section seems out of place.
I consider myself white, but I suppose at one point I would have been considered Irish? Maybe?