In the US, the best, and really ONLY, place to go is to the source of knowledge itself: http://www.copyright.gov/.
Any other work will be less accurate than this legal body who makes the rules in the first place, and like everyone they revise their work based on new information and the needs of the public.
Don't worry, they have plenty of things for you to read and download, and FINALLY (after years of begging by those of us who write electronically) they even have a section on electronic copyrights ("eCO").
While I'm here on the site, I'll brush up on things myself...
Great question to ask, especially since copyrights affect all of us in one way or another. Thank you!