A couple of years ago a new cookbook by Diana Kennedy on Oaxaca was published in Mexico. I've got it in Spanish and it is quite a tome. The original publication was only about 3,000 copies and financed - I was told - by an industrialist in Monterrey, MX.
University of Texas Press will be publishing the English translation in Sept. 2010. Here's a link for more information - http://www.utexas.edu/utpress/books/k...
It is available for pre-order and there does appear to be a pretty hefty discount for doing that.
It's a hard cookbook from which to cook in the U.S,. (and perhaps even in Mexico, too) because of the specialty nature of some of the ingredients. But as a recorded history, so to speak, of the cooking and culinary traditions of the different regions of Oaxaca, it is probably a reasonably good reference. Don't be put off or deterred by the "coffee table cookbook" appearance to this book, it's a pretty serious work.