Heating systems are all about controls. Much of what you need to know is on the manufacturer's instruction sheets for the various controls. Honeywell, Mcdonnell-IIT, etc. Power comes from the transformer, through various and sundry controls, then to the ignition module for whatever the fuel type is. Not so hard, really, when you break it down like that. You can have other accessories like water fillers and zone valves, too. I'm not aware of a book on heating systems particularly, but there's lots of good information in the "Modern Air Conditioning and Refrigeration" book by Althouse, Turnquest, and Brachiano.
THere's not much to the line voltage portion. Basicly just landing power someplace. Sometimes you need to get 120 volt power to certain accessories, like power venters and such.
