Donate Books
One of the easiest ways to get rid of your old books is to donate them. Call a non-profit organization like the Salvation Army. They will send a truck to pick up your old books. You'll get a receipt that you can use to claim a tax deduction.
Used Bookstores
Sell your book to used bookstores. This option requires little effort, and you'll walk away with cash in hand. The drawback is that used bookstores only pay a small amount of money, even for books that look new.
Sell Books on eBay
You can also sell your books on eBay. However, I prefer selling books on Amazon because I have had more success with it. This is mainly because there's more demand on Amazon, and you can use metrics like the Amazon Sales Rank (BSR) to determine how likely it is that a book will sell.