The Happiness Project is a great, uplifting book to read when you are feeling down. I also find that Barbara Kingsolver's books will lift my spirits, with their funny and loving characters. One of my favorite books by Barbara Kingsolver is The Bean Trees. I would recommend that book to read when you need to feel better about yourself and the state of the world.
There are also thousands of self-help books to read if you are feeling particularly down and are looking for concrete solutions to feel better. Many self-help books can be availed in the form of e-books from online stores.