Lots of people have suggested books, and articles, but I'd like to add (internet) photography forums, or even just photography sections on other forums. If you find one which has users with portfolios you like, and the people who post there don't seem too up themselves, then you really lose nothing by joining and posting.

These create a really great was for you to get live constructive criticism, and help you develop your own style and sense of photography, eventually leading to learning how to critique yourself, and when to see if the critique is objective or subjective as you get better and better.