Edwin Haislet's book is excellent reference material, and it goes over the fundamental footwork pretty well. (http://www.lulu.com/items/volume_64/...in_Haislet.pdf)

greynotsoold has several posts that go over footwork. One post in particular that I recommend that you look at is "Coordinating the hands and feet", which you can find in the useful/important posts section. It's a guide on putting your punches and footwork together.

As for some thoughts behind footwork, Andre and ThomasTabin have some excellent information on the subject. See Thomas's "Circling", and http://www.saddoboxing.com/boxingfor...ootwork-4.html which has alot of things to think about and practice. Check out Andre's article for more: http://www.saddoboxing.com/andre.html

It's all about understanding what you're trying to accomplish with your footwork, and practicing those movements until it all becomes second nature. Practice it while you're shadowboxing while trying visualize where you're moving in relation to an imaginary opponent. When you're hitting the bags, don't stay in one spot. Bags don't hit back, but you should pretend that they do. Imagine a counterpunch, choose how you're going to avoid it, and where you're going to end up.