Sure, it's a good idea, and if you do a google search, you will find that, as you say, it is not new, and is being done now. It can not only link to a website, but it can also open up an interactive communication (of course textbooks sometimes do this, but think what Rowling did with her interactive websites). You could provide up-to-date offers, or even (if you had a small target audience of specialists or were a bit masochistic) allow the readers to communicate directly with you.
I think its a great idea. If someone thinks its ugly, it will just be skipped over. For the people that know what may be hidden behind a QR code, well, there you go.