Here's my current status and opinions about Mexican cookbooks.
I still like Rick Bayless's Authentic Mexican and even if you only get one Mexican cookbook, this would be an acceptable choice because it's usable and good.
Diana Kennedy's books are more comprehensive and better overall, but a little more overwhelming if you've never tried to cook much Mexican food (no, it doesn't count if you've dumped taco seasoning mix into ground beef). I have her Essential Cuisines as a real book and I really like My Mexico. The first is a good choice if you're only getting one Mexican cookbook. I think the second requires a bit more experience. And if you've traveled around Mexico some? It's wonderful.
The other book I'm using all the time is David Sterling's Yucatan. This book is beyond wonderful. I got it before we went to the Yucatan because it looked like the perfect combination of a travel book and a cookbook, and it is (and I got it so I wouldn't be totally clueless when we had to eat there). It's not for a beginner though unless you're ready to tackle a challenge, and it really helped to go to the Yucatan (not Cancun, mind you) to see what goes on there with food, but this is one of the best Mexican cookbooks out there, no question.