I don't know where you live so what you think might be authentic might not be so authentic. Depends heavily where you live.
Mexico is also quite regional so if there are regions you prefer over others, then that will inform which books to pick. I'd probably start with your local library and work through what they have on the topic to get some background in the cuisine and where you might like to learn more.
http://www.mexicocooks.typepad.com/ is a handy website and lots of book ideas there too.
I've liked a lot Bayless's work but he tends to pick the variants rather than the common. All the same, Mexican Everyday is a very good book, even if you never cook from it. He did more in that book to help me understand shopping than Michael Pollan ever dreamt of. He also makes a good point that much of his early work (and lots of other cookbooks as well) were more about event food than what gets eaten as the staples of the cuisine. And that is the heart and soul of the cuisine.
Diane Kennedy is good, certainly.
If you want to look into texmex stuff, I like the Borderlands cookbook by Jamison and Jamison, but it's a little sterilized. Good starting point though.
Daisy Martinez is of Puerto Rican heritage but you can get a lot of mileage from her book Daisy Cooks on Latin style cuisine.
Steven Raichlen did an enlightening book on Latin Cuisine as well, though I didn't like the recipes. Steve Raichlen's Healthy Latin Cooking. He discusses how the core foods of Latin cuisine can be stunningly healthy, great fiber, lots of vegetables, fresh fruits, inexpensive but that what gets popularized is loaded with added fat and dairy, and probably fried as well. And I love that too, in fact refried beans just sing with bacon fat. As do red beans and rice,... I think he missed his own point in much of the recipes swapping in low/no fat fabrications rather than on making the best of what the cuisine could be naturally. Still worth a look but not a cook.
What ethnic grocers do you have access to? They're a tremendous help in finding specialty ingredients and saving money on them. Because that too will influence where I think you should start.