Here is a good website to add to some listed above:
http://autonopedia.org/crafts-and-te...achine-repair/
I have taken Ray White's class, and even though it is expensive, it is worth every penny. He just knows everything there is to know about how sewing machines work! I also read TreadleOn every day, and that has contributed immensely to my knowledge. Sewing Machines are just neat little things. The way they work fascinates me!