Have you heard of Swaptree? I've know about it for a while (actually, a friend of mine, Greg Boesel from B-school founded it), but I've finally started really using as my primary source for acquiring new books and music. There's a good selection of yoga books...but the more of us who start using it, the more cool titles that will be available.
It's like shopping on Amazon for books and music...but for free! Click on the books/music/dvds you want, and enter in the books, etc. that you have and are ready to part with (remember the yoga principle...non-coveting?!) and swaptree matches you up with people to trade with, gives you their address and a mailing label, and that's it. The only cost is shipping, and fortunately media mail rates are really low. So I invested a few hours of my time, to populate the items i own and want to trade...now I can swap my items with folks around the country.