If you're not familiar with BWB, go check out their site. We think you'll be as impressed with this company as we are! They describe themselves as "a for-profit company with a social mission where every purchase you make helps to change the world."
We send our leftover books to them and they in turn try to sell those books online; you may have seen them as a seller in various used-book marketplaces online. If a book sells, the proceeds are split between BWB's administrative costs, a small payment to our library, and a donation to a literacy partner organization (ours is Worldfund, an organization that supports students in Latin America). Books that can't be sold are recycled; BWB prides themselves on never sending anything to a landfill.
Better World Books sells both new and used books (and they throw in free worldwide shipping). AND, for every book sold, they donate another book to someone in need. Pretty great, right?
So how can you help? Well, you can donate books (they pay the shipping for your donation, too). You can sell your old textbooks through BWB. And, when you're buying books, either search directly through the Better World Books website or make a point of choosing them in the Amazon, Barnes & Noble, or Half.com marketplace.