Ok, given that B&N have made two more structural changes since my last post and the current state of play is an inconsistent mess depending on the book I'm going to give this a day or two to settle down before I publish the next version. I don't know how they do their release process but it clearly isn't anything close to what I would do with a commercial app.
One minor unfortunate impact of their changes is that the plugin now has to do an extra hop to get the large image (if it exists). Maybe they did that intentionally to discourage robots etc. However in its current state it also means that when you click on the cover of a book on their detail page you get a popup window that is blank when they have no image. Utterly crap, so much for their testing process.