The morning bus commuter next to me today was reading 'The Geography of Bliss' while I embroidered a quote. She politely inquired on my work and I on her book.
She told me about this book swap site, free at this time, for anyone who wants to trade:
It's not just for paperbacks, but hardbacks and audiobooks. It's great for kids books.
If you sign up 10 books you are willing to swap, you get 2 points to start. For each point, you get to request one book from someone. When you request a book, it is mailed to you (free of charge). In return, when someone requests one of your books, you mail it to them (you pay the postage). Mailing costs are pretty minimal, maybe a couple dollars a time.
She is a regular, and really recommends this group.