Have you sent the books? If not, if you cancel the orders, they will get their money back and you will still have the books. You may still need to pay some fees e.g. if you used a reserve I think.

If you've sent the books, then you can discuss with the buyer. If you cancel the orders, it's possible you never hear from them again (and ebay will not take any action, as you've done the cancellation). Or maybe the buyer will buy the same books again - (I suggest) best done in one item.