Giving books to charity is a great way of getting rid unwanted books (Or less liked books) Charities are meant for people in need, so you'll know that the book will get to someone who will be grateful for it.
If you don't know any charities you could just google it to see if there are any close to you. The red cross usually takes book donations
give to a friend
You might not have liked the book but maybe a friend will? Free books are usually always welcomed among friends, and it's a way of making sure that the book actually becomes "recycled" Who know, maybe your friend will love the book and you to can have a debate about your different opinions
use as decoration
Having nice looking books around the house works GREAT a decoration! Especially in the hallway and living-room :) I love stacking some books and then putting a vase with roses on top, gives it a romantic feel. (Valentines day are coming!)
use for DIY's
There are tons of DIY's you can do with books, just google book diy or something and you will get many hits. A favorite of mine is the secret pocket book diy. All yoy basically do is cut of square pieces from the pages so that you have a hole, and then you can put things there and no one would know ;)
donate to public/school library
This title pretty much says it all, libraries loves getting books and especially school library's. They are a bit picky with the shape of the book though.