If you consider yourself green, used books are the perfect example of re-using and recycling.
So stop buying the newest, the latest from Borders and Barnes and Noble. Don't rush to Amazon to find the hottest deal. Walk to your local used bookstore, pick up a book you know nothing about, catch up on all of the Pulitzers, search for neglected masters like Anne Stevenson or Conrad Richter or Robert Lax, hunt out the women and minorities that don't always make the newest and hottest list, attempt the Koran, look at Buber and the Bible, consider spending some time with a good copy of Gilgamesh, find a modern library edition of James Joyce's Ulysses and start your own James Joyce reading group.
If you must buy online, turn to a company like Powell's or Biblio.com. We list on Biblio.com. They've just become the first major book marketplace to be carbon neutral. They also have a non-profit arm which builds libraries in South America.
Okay, I admit we list on Amazon, too. That's where most of the sales come from, but I'd much rather see you click on the biblio link in the sidebar. If you do, use coupon code BEGREEN to receive %10 off a purchase of $20 or more from Miss P's inventory.