This seems like a pretty interesting book reflecting the social angst that came out of Oaxaca, Mexico in 2007. At the time, Mexican police officers opened fire on a group of protestors who were advocating for the right to an education. A strong social backlash was channeled through the art form of graffiti, and Protest Graffiti Mexico: Oaxaca is comprised of images of the graffiti photographed by, the now deceased, Elaine Sendyk.
You can purchase Protest Graffiti at Amazon for $21.24, and there are some really insightful comments about the book at BleedTheSoul.