I’ve recently joined two book trading sites.  Both work about the same way. You establish a free account, then post books that you are willing to trade. Through my LibraryThing account, I can see which books are in demand on these sites. When you agree to send out a book, you pay for shipping. But, you earn a point to exchange for a book (1 point = 1 book). When you request a book, the sender pays postage, and you get it for free. It’s a great way to refresh your library. I was surprised by which books in my collection were in demand, and there were several I was happy to send off in exchange for points. The sites I’m trying out are Paperback Swap and Book Mooch.