As much as I am thoroughly unimpressed with RMS these days, the man has done a lot of meditating on the nature of copyright. His Free Software Free Society book is a good read on the subject from his point of view. It is also freely available under the GNU FDL I think.
Lawrence Lessig’s Free Culture and Remix would also be good, the former tracking how copyright has changed and mutated since the 70s. Both under creative commons licenses I believe.