Do you know about Bookmooch? It's a free website that allows you to list your own used books, and then "mooch" books you want from others. If you send a book, you get a point. You can use that point to get a different book. Simple, and yet genius! The only cost involved is mailing your books.
I usually take my brown grocery bags, cut them up, and wrap the book in it, and it's typically $2.33 to send.