One of my greatest discoveries was that of www.PaperbackSwap.com. I learned of it when I was pregnant with Beads about 5 years ago. This is a service where you can post the books you own and are willing to part with. When others request your book and you send it to them, you receive a credit. These credits can then be used to order books from other members on the site. I can find any kind of literature here. There are literally millions of books available, and if a member does not have the book you're looking for, you have the option to purchase the book from Amazon.
The beauty of the site is that it allows you to pass beloved books along to people who really want them. Donating books is a good and noble thing, and there are many organizations who are grateful for these donations. However, there are times when I have carted books off to good will and wondered if they really ended up on someone's bookshelf or in a recycling bin. On the flip side, it's an economical way to find books I've been wanting to read, so that I'm not breaking the bank on frequent book purchases.
If it's time to clear the shelves and create a little breathing room in your home office, this is a great way to start.