Free Culture by Larry Lessig.
A free online version of the book is also available to download, as a Wiki, and in audio all under a Creative Commons license.

Excerpt from the NYT review: 

The biggest issue in intellectual property today is how to handle Internet file-sharing, and Lessig has some interesting thoughts. Most analyses wrongly lump all file-sharing together as piracy, he says, when there are four distinct types:

a) downloading content, like a Madonna CD, instead of buying it;

b) sampling content before buying it;

c) downloading content that is no longer commercially available; and

d) downloading content that is not copyrighted, or that the rights owner wants to share.

Only type d) is currently legal, but Lessig contends that b) and c) do not do any harm. The Napster problem can be solved, he suggests, by finding a way to deal with the harm that type a) file-sharing does to copyright holders.

Professor Lawrence Solum has also reviewed Free Culture.