I generally look for cookbooks that have a good deal of explanations of techniques or thinking behind the recipe. There are too many cookbooks published with pretty photos and just a bunch of recipes. This makes it easier for me obsess over every cookbook that comes out with some famous chef on its cover.

The cookbooks I use the most often are:

French: Julia Child's Mastering the Art of French Cooking Vol 1. Pepin's Complete Techniques (combo of La technique and la methode), and Bouchon.

Western/other: New Best Recipes (Cook's Illustrated compiled essentially), More Best Recipes.

Chinese: Fuscia Dunlop's "Land of Plenty", Dunlop's "Revolutionary Style Cookbook", Pei Mei's Chinese Cooking Vol 1 & 2. The latter two are well worth finding from a used book store or used on amazon.com.

Indian: "50 Great curries of india". I don't have a good sense yet of good Indian techniques, but the explanatory chapters in the beginning of this book were pretty good.