I found the checkout process needlessly confusing. The split between the first and second page once I clicked was not clear to me, and the name of the button beside the price box was "Gift". With the arrow above it specifying a price, I didn't understand if I was entering the price of the book, or an additional sum. Could I have paid less and still received the book?

Finally, once I have paid for the book, it seems I can't get back to the page again, so I guess I'm not supposed to be able to buy another copy?

I would look over the usability of the checkout process, I almost didn't buy it because of the perceived ambiguity. And I would have preferred to use Paypal.