This post is going to sound like a paid endorsement, but I promise it isn't. I'm just enthusiastic about this website.
If your bookshelves are overflowing and you want to find good homes for some of those books, I have the answer. If you're always looking for something new to read, I have the answer. The same website is the answer to both situations -- Paperback Swap. Don't let the name fool you; you can swap paperbacks, hardbacks, and even audiobooks.
Here's how it works: you list the books you'd like to give away. The site makes it easy to list these. After you post the first 10 books you'd like to give away, you get 2 "startup" credits so you can start requesting books. After that, you earn credits by sending out books to people who request yours. You can order one book for one credit and you receive one credit for each book you mail. The only exception is audiobooks; they're two credits each. The only money that's involved is the postage you pay for when you mail a book; you can send it by Media Mail, which is usually cheaper than First Class. Sometimes with thin paperbacks, First Class is actually cheaper.
There's an easy "search" function to find a particular book you want to request. I've gotten some obscure books as well as popular ones. If a book you want isn't in the system right now, you can put it on your "Wish List" and you'll be notified when it's available.
After you've read a book, you can keep it or you can repost it and eventually send it to someone else. There are many, many books I've reposted and some I'm keeping forever.
I can't say enough about how great this site is! You should see the "To Read" shelf I have; it's full. I don't even "look" for books anymore. I keep getting books from my Wish List as I move up in line. I keep sending out a lot, too. Doris at the Post Office Express knows me quite well by now.
Start swapping those books; it's a lot of fun.