Nikon has something called a digitutor that helps you learn. I'd recommend Bryan Peterson's books, particularly Understanding Exposure as a first book to read, but it's also great for people at any level of experience. The photos are beautiful, and you learn a lot in a very short amount of time. He also has online courses which I hear are great. Someday I would like to take their classes on people and pet photography.
Understanding Exposure, 3rd Edition: How to Shoot... Pet Photography Understanding Exposure and Your DSLR Portraits Unplugged by Bobbi Lane