I struggle with clutter when it comes to books. I have to make sure I get rid of books and not just acquire them without end. There are only so many bookcases that will fit.
Paperbackswap has been a great way to turn books I no longer want into books I do. Despite the name, it traffics in books of any binding (audiobooks, too.) Its guiding assumption is that all books are created equal: by sending out a book, at your expense, you acquire one credit. You can use a credit to request a book from another member, at that member's expense. (Audiobooks are worth two credits.) It relies on the U.S. Postal Service, so it's only available for people served directly by the USPS.
To join, you post ten books (in acceptable condition) that you're willing to part with. You get 2 credits just for doing that. If a member requests one, you have two days to confirm, and then two days to mail the book (nominally -- the system will only cancel the transaction if over a week passes.)
You can define your own conditions regarding the books you'll accept; the other member will see your conditions and can refuse your request if the book doesn't meet them. At first, I had "no books from smoking households" as a condition, but I dropped that. The much more frequent problem was books that smelled of perfume. Excluding books from perfume-using households seemed overly restrictive, and it's easier to get the smell of smoke out of a book than perfume.
The availability of a given book is hit or miss, but, if it's not available, you can put it on your wish list. Whenever that book's posted, it's offered first to whomever wished for it first. (Likewise, when one requests a book that multiple members offer, the request goes to whomever listed it first.) Wishing for things is key to getting the most from the system. For instance, there's almost never any Terry Pratchett available, but I've received a half dozen Discworld novels through my wish list (and patience.)
To send a book, you print out a wrapper provided by the system. For an extra $.43, you can print the postage and get delivery confirmation. If you do this, you receive your credit when you tell the system you've mailed the book (otherwise, you don't get the credit until the recipient marks it received.) A package bearing stamps and weighing more than 13 oz. (which a lot of trade paperbacks and most hardcovers will exceed) needs to be mailed in person at a post office. The printed postage saves you the trip since it's not bearing stamps. I became a happier user when I switched exclusively to paying the surcharge for the printed postage.
Shipping a book typically costs about $3, so the books you receive cost you $3 and a book you didn't want anymore (plus your time and effort to wrap and ship it.)
If you join and say zedlopez referred you, I'll get a credit. But I recommend it on its own merits, not in hopes of scoring a book.