Updated from hardcover version Sep 6, 2010
by Andrew S
The paperback edition of John Rawls' classic contains a new introduction and a "Reply to Habermas"--both of which are widely cited. The paperback edition is therefore worth getting not just for those who don't have the hardcover edition but for those who do.
I'm not going to write a review of the book itself because it's so famous; those who want to know more should consult the Stanford Encyclopedia of Philosophy or such.