Better World Books is THE best place to buy books online. I placed my first order with them in 2008 and I consistently turn to BWB any time I buy a book online. They sell both used and new books and the used books are sold at insanely great prices. You can often find books in the $3-$4 range and shipping is free. FREE shipping!
The only catch is that shipping does take QUITE a while, so if you're on a deadline, it's not the best option. However, if you're looking for books to leisurely arrive on your doorstep, it is the best option for online book shopping.
The coolest part? The money you pay for books goes to literacy programs around the world. They've raised over 15 million dollars for literacy programs and libraries around the world.
Love them and highly recommend them! This post is all on my own - no compensation whatsoever from BWB!