Modern Welding is a comprehensive text that has long been the standard for teaching the theory, fundamentals, equipment, and techniques of welding technology. In addition to covering a very wide range of welding and cutting processes, the text includes thorough coverage of welding symbols, testing and inspection, and getting a job in the welding industry.
When they say comprehensive, they mean it. Assuming you can process this tome, you'll know probably more than you'll ever need for a job, theory-wise. With the proper technique to back it up, I'd say a strong familiarity with this text would put you in a pretty good place to instruct welding courses. I'll admit I skipped the sections on laser welding, explosive welding, & various other special processes, because well, I'll probably only pick up an Oxy-acetylene torch or arc equipment, but hey if you're interested it's all in here.