There's a cool article about these codes in MAKE's sibling publication Craft, vol 02. People use them to swap Creative Commons t-shirt ideas at
http://cshirt.sargasso.jp/ -- you can scan the code on a real-life tshirt created there, go to the site, find the same shirt, and print it as-is or make a remix. The site is in Japanese, though, which can be somewhat problematic for gaijin such as myself (I read a little bit of Japanese, but not much or quickly). I think I could probably figure out how to create a shirt, though. Too bad the QR-enabled phones aren't sold here.
Back to the subject of the thread, I don't think the QR codes hold enough data for a whole book-- YET. Possibly they could, in the future. It's an interesting idea. They could certainly hold enough info to identify a CC type book on a site like this, though. It could easily be combined with a POD system for those who like physical books, or the card identifying the book could be sold with a PIN covered by scratch film. If QR phones or their equivalent were common in other countries, one could buy a card, the recipient could use the QR phone to scan the book ID, the ebook site would ask for the pin, then deliver the ebook to the phone (which is where a lot of people read their books, I guess). An account would also be created that would allow the same user to download the book again to other devices. One side of the card could hold the QR code and a bit of blurb about the book, the other side could be printed with the book's cover illustration. I think this could work quite nicely -- if QR phones worked here.
Of course, one could simply type in the ISBN from the card and get the same effect.