There are some very knowledgeable people here, so definitely read through some threads, but when I was just starting, books from the Monks of New Skeet were amazing.
The monks are in the US, and as part of their spiritual practice, they train German Shephers and rehab dogs of all breeds.
I highly recommend their book How To Be Your Dog's Best Friend. It covers not just how to teach specific commands, but stresses the importance of exercise and incorporating your dog into your routine.