To archive them you press and hold on the title that you want to go away. It then gives you a pull up screen with the option to archive the item. It took me a while to figure this out...
I find this slightly lame. There should be more options for trading, selling etc.
This is why I try really hard to limit myself to library books. It makes me feel a lot better about reading so much when I don't have to pay anything for the books. It feels worse to me now to buy books than it did before, maybe because I can't give them to people.