Good friends spent Christmas in Oaxaca. They lugged back 3 copies of the newly released and updated version of Ricardo Muñoz Zurita's Diccionario.
Not a cookbook, but probably the best reference and resource out there on Mexican cooking and cuisine. It covers just about everything from A to Z in 644 pages. In it Ricardo defines ingredients, cooking styles and methods, the cooking in various regions, utensils, festivals, foodways and more. There are also lists of antojitos, moles, mushrooms, chiles fresh and dried in handy dandy chart form that make it easy to figure out where a dish originated, what the major components are and what it might be called in the various areas of Mexico.
This is an update of his earlier (10+ years ago) Diccionario and was published by Larousse. They've done a bang up job with the new edition. It's beautiful, well laid out, easy to use and enhanced with many color photos.
This is a tremendous resource for serious students of Mexican cuisine. Yes, it is in Spanish.