If you want authentic Mexican cookery, then try any of Diana Kennedy's cookbooks. But "Tex-Mex" is a hybrid of Mexican cookery, adapted by Mexican immigrants in the US to incorporate available ingredients, and has many features that Mexican cooking does not, e.g., cheddar or jack cheese, lettuce. Maybe Rick Bayliss?
If you google Tex-Mex recipes, you'll get a lot of hits.
The only cookbook I can think of is "Best of the Best from Texas Cookbook II". The blurb on the back says "Treat yourself to the big, bold taste of Texas...it's Tex-Mex Plus!" It is available at Amazon.com or Quail Ridge Press at Quail Ridge Press - the BEST cookbooks & FREE recipes online. There are other recipes in the book as well.