[gms7777]

I really like Mark Bittman's How to Cook Everything Series for this. It has a ton of recipes, but there is also a decent amount of discussion of the concepts behind recipes and multiple ways to alter most recipes, as well as tables that make this sort of concept explicit (e.g. There is a table for soups that has a list of well known soups with a column for the liquid base, protein and vegetable). I rarely pull recipes directly out of this book, but it has completely changed the way I think about cooking.

[jschulenklopper]

Second that advice for Mark Bittman's book. The main advantage (for me) is that it briefly discusses the basic recipe (and some of the reasoning behind it), and then mentions 10-20 variations w.r.t. ingredient alternatives that are worth a try.