I second the vote for Jamie Oliver's
Ministry of Food. It has some really quick easy recipes and some that, while easy, take hours and hours to make (let simmer...) and are completely worth it. It's the one we use the most.
The most interesting cookbooks I have are the Alton Brown ones. His regular cookbooks (I'm Just Here for the Food; I'm Just Here for the Food: Kitchen User's Manual; I'm Just Here for More Food) explain a lot about the science of cooking - how different cooking methods change the chemical composition of food, how to change your results by changing little things; the advantages of different cooking utensils (he believes very strongly that cooking gadgets should all be multi-functional). The TV show cookbooks (Good Eats: The Early Years; Good Eats: The Middle Years) cover the highlights of each episode, informationally and a couple of recipes. If you haven't watched Good Eats, I recommend it. They're a lot of fun.
My favorite, however, is Nigella Lawson's
How to be a Domestic Goddess. I love baking!
Also worth a mention is Epicurious. There's
http://www.epicurious.com/ and an iphone app and years worth of recipes from back issues of cooking mags. You can search by ingredients, too.