Taco USA: How Mexican Food Conquered America by Gustavo Arellano.
It gives an excellent account of the story of American's love affair with our neighbor's gastronomic legacy. There are also chapters devoted to the legitimacy of Tex-Mex and Southwestern cuisine, which could be used to settle some long-standing arguments. I found it fascinating, most notably the fact that "Americanized" Mexican food and the products associated with it weren't started by ignorant gringos but by immigrant entrepreneurs who's innovations helped solidify the definitions of Mexican food in the US. Many of the prominent chefs, authors, and other food personalities associated with the cuisine get a mention: Rick Bayless, Diana Kennedy, Susan Fenniger & Mary Sue Milliken, John Rivera Sedlar, etc. they're all in there. A great read if you love food history.