Tax law is a research hydra (though hopefully less bloody). It's comprised of multiple 'heads,' all of which are important and necessary to search. I'm sorry the only analogy I can come up with is monster-slaying. That doesn't bode well, does it?
The primary focus of this guide is federal tax law, but foreign and state taxation will get their moments in the sun (i.e., mentioned and links to some of that stuff, too).