Used books have, in most cases, but a little time before they become either "classics" or end up on the "price reduced yet again!" table. But, now and then, a movie comes along, and, surprise! a really good book, like Georgiana: Duchess of Devonshire, by Amanda Foreman, comes roaring back. And those used copies in hardcover and paperback are looking good again and going fast.
Look for new and used copies, before or after seeing the new movie "The Duchess" staring Keira Knightley.