I try the same practices when it comes to other consumer goods, clothes, and crafting materials. But sometimes we just have to buy new. This is because it is a newly written book, or a new invention or consumable materials.
This is where it becomes a more difficult road to traverse. Adding up food miles, buying locally made, looking for the phosphate-free, or biodegradable alternatives, choosing less packaging. It's a minefield and sometimes it's harder for me than simple recycling because I was raised by parents who composted & recycled as much as possible, long before it was fashionable.
Well, thanks to Eco Libris they have provided us with some ideas on that score. Through their site you can plant trees to balance out your books. You can learn about which publishers are using environmentally friendly techniques and they have also started the "Green Books Campaign" which is linked here.