Here is an excellent web video series called Everything is a Remix that explains the history of derivative works and copyright. It is an excellent series. Check it out!
Part 1:
Part 2:
Part 3:
Link to Episode 3 Soundtrack
Part 4:
Link to Episode 4 soundtrack
Along with these videos, an excellent bibliography of samples and references has been provided, if you're interested in the content that was shown. It's very impressive.