For when the library just won't do...
We gave our boyfriend a used book for Christmas. We couldn't rationalize the waste of paper for a brand new version, but we also couldn't help but feeling, well, a little cheap?
We debated the decision, ultimately deciding he'd agree more with the green purchase. (Which luckily, he did.) There's so many outlets now besides your local book store, even Amazon and Barnes and Noble let you purchase used copies.
They're not only cheaper, but you can often see the seller's location, so you can choose someone close to home to reduce shipping distance and costs.
While we might think twice about giving a second hand novel to someone we don't know that well, (and knowing that the author won't benefit from the sale) for us, used is still the way to go.

Commercial Flour Sa...
I love used books. In fact, the only time I buy new books is when it's a new release that is either hard-to-get (some of my favorite authors are British without wide distribution here) or one I've been anticipating and would like to read at a certain time - the last Harry Potter, various plane books...
Used books have a sense of history. And they smell better. :-)
http://embritadesign.blogspot.com
My favorite online bookstore that has TONS of used books and rare books: Powell's in Portland, OR (http://www.powells.com/)
Their entire inventory is listed online, and they use clear descriptions when listing the condition of the book. What's better, if you click on the condition description of a used book, you get a pop-up describing the definition in more detail. They're easy to use, and have awesome customer service.
If you want to be really green, and support a local business, look in your phonebook- there are used bookstores in almost every town I've ever been in! They might not have the selection of Amazon.com, but you can call ahead if you are looking for a particular book, and who doesn't love some time in a good used bookstore?
I used to volunteer for our library's used bookstore, which paid for children's programs with the proceeds. Incidentally, I gave and recieved one used book each for Christmas.
I only buy books new if I can't find a used copy anywhere.
I collect books on fashion, and since so many of the best are out of print, most of mine are used. If my boyfriend gave me a used copy of Elsa Schiaparelli's autobiography, I'd be proud of him for tracking one down, not insulted that someone else had owned it for a while.
Many of my books are used, too! If you want to make a great present to a bookworm consider an antique first edition of a book.
Three words.....LIBRARY