Based on the Amazon description, I'm not sure most people would be interested in downloading the ebook, but the trick used here is worth copying by other self-published authors. Let me post the unclipped photo. It will give you a better idea of where this sticker was stuck.
It might be hard to picture the location, but the sticker is placed so that anyone sitting and using the porta-potty can't help but see and read it. This makes them a captive audience, and that's always a good time to try to get someone to notice your ebook.
I'm not interested in reading the ebook, but I did want to bring this idea to everyone's attention. It's something that I think more authors could copy and use to promote themselves.
First, it's important to note that this isn't a new trick so much as it's an adaptation of existing techniques. While I've never seen a QR code used in quite this way, I have seen authors and publishers leave everything from postcard sized summaries to complete print books as a way of catching the attention of a passerby. This type of marketing is particularly common at BEA, where there's so much discarded stuff in the lobby it rises to the point of being littering. (I think i like this one better due to the simple fact that I don't have to wade through the debris.)
But as great as this idea is, I also think it can be improved upon. A captive audience is always a good thing, but before you copy this you might want to rethink his description. You don't want to waste someone's time (particularly if they're paying for the 3G data plan by the MB), so it might be better to offer at least a short description of the ebook in question. I'd also include the title, just in case the next person who sees it doesn't have a QR code scanner. I myself didn't have one until i downloaded it so I could scan that photo, and a lot of my Android devices don't have a camera so a QR code wouldn't do me any good.
Speaking of adding more information, this program (or this one) might give you some ideas about what you might include. Each of those programs used a big sign to get people's attention, and that's a little beyond what most authors can afford. And if you tried to shrink one of the signs down to a sticker it would be illegible, but I still think that there's a happy medium between the sparse sticker above and the signs used elsewhere.