I make this my number one because I use it so often. The prices for brand new books are reasonable and the shipping is $3.99 for the first 1 or 2 items and then $.50 each additional item.
Currently BookCloseouts is having a sale in which their entire Scratch and Dent category (the used books essentially) is 30% off. That means... books that are normally $2.49 are now $1.74. Not bad for a book, eh?
2. Better World Books
This is a site where your purchases are donations sent to various libraries and literary campaigns. Better World Books states "library books are transferred to Better World Books under consignment and sold with a fixed percentage of the net revenue paid to both the library and a non-profit literacy partner. Typical arrangements are 15% of the net sale price to a library and 5% to their chosen literacy partner." That makes your purchases not just a contribution to a library but also a literacy campaign to promote reading. That is a helluva reason to buy from them! In the world today, how many organizations actually cut their profits to benefit non-profit organizations? Not many.
Currently they are also having a promotion in which 3 books from the Bargain bin are being priced at $10 or 4 is $12. Each additional item after that is $3 a piece. This is really good for hardcovers, but keep in mind these ARE used books. They are ex-library books for the most part. Make sure to keep an eye on what quality book you are purchasing (Very Good, Good, Acceptable, etc.)
This site is not completely focused on books. There are also CDs, DVDs, Games, Comics, Action Figures, and much more. This is an inexpensive way to stock up on ALL your entertainment devices (yes, they do have e-books too). Their shipping is $.96 for the entire order.
Those are just 4 off the top of my head. After I do some more research and find some more sites I will be sure to post them so you may purchase some more of your favorite books. I am an advocate for used books. I believe that just because a book is well read that they shouldn't be added to a collection. A book does not have to be pristine to hold a world of magic, mystery, romance, and wonder. Show some love to used books.