This might not be a new idea, but I've decided to put a QR code on my the back cover of one of my books. The code itself is a simple link to the official website for the book, but there are many possibilities with this. You could have a link to an online shop, or a sample for people with smart phones to take away and read. The only downside is the appearance - some people think QR codes are ugly. I might be a few years behind the times, but some people might think they're cool. Do you think QR codes are a good thing to put on a back cover? What information might you want to hide in there?