The basic premise is that any system is governed by multiple influences. Lessig identifies four factors.
Each factor influences and is influenced by the others. Lessig sees code (technology) as a very strong inhibiting influence on the unfettered power of the market or law.
The book covers a wide range of topics within this area, from anarchy (using post-Soviet Russia as an example), anonymity, pseudonymity, privacy, and free speech.
Very highly recommended. Required reading, in fact.
Lessig is working on an updated version of the book, and is using a JotSpot wiki as an experiment in online collaboration: http://codebook.jot.com/WikiHome.