I guess that for me, cookbooks are a step-by-step guide for making something completely new. After the first, or maybe second time, I'll never look at the recipe again - the knowledge is already inside of me. I feel fortunate enough in my experiences to have tried a wide range of cuisines from around the world.
The ultimate educational tool, for me, has been watching Good Eats with Alton Brown over the last.. I don't know.. five years? Knowing how each ingredient works is such an efficient means to get whatever is in my head to come out. If I don't have the knowledge, of course I will lean on a recipe one or two times, though a cookbook will go mostly unopened for me - and I'd rather have the space to put a new kitchen appliance or tool.
I used cookbooks for years - like I said, it's how I learned how to cook. But I've found that I don't open them - making the internet and Alton Brown better tools for me.
Haha, checking out the Test Kitchen link that Steph provided... now
that's a cookbook I'd love!