I'm going to assume you already know the basics of cooking and are looking to expand your repertoire? I use blogs a lot, probably my favorite is Smitten Kitchen. I think I've made over a hundred of her recipes and I've had one flop. She has a book too. Serious Eats and Budget Bytes are good too. For Asian, particularly Chinese, Jaden Hair and Bee Yin Low's books are good. I like Dorrie Greenspan's baking book. I have an ancient Betty Crocker from the 70's I use a lot for baking recipes. Marcella Hazan is to Italian cooking what Julia Child is to French cooking and her Essentials of Italian Cooking is fab.
I am gaining more and more knowledge about Southeast Asian food since my gf is from there and this is kind of what made me want to pursue cooking as a hobby. Chinese food would be a good skill to have. Especially some beef, pork, or shrimp lo-mein.
I am interested in a lot of Italian and Mediterranean foods too. Pastas, aranchini, lamb kabob or gyros, etc. I love tzatziki sauce. I am already pretty good with most american foods and somewhat good at Mexican/TexMex foods - nothing amazing, but better than chipotle or qdoba. Basically, American and Mexican will be a lower priority since other areas are lacking more.
I don't care much about baking or desserts.
To everyone else, thanks a lot for the suggestions! I will have to review them all and check out the books/blogs.