They are very easy to add to your book if you are publishing yourself or run a publishing company. And while you will probably always provide a print web address - they make a nice addition e.g. for people in bookshops who may not buy there and then but want to get more information; or need to confirm. They can just scan, read or store the details and job done.
- Decide on the information you want to encode - ideally this should be a page that knows it is being viewed by a mobile device with a small screen and adapts itself; at the very least a page that users can use on any device.
- Encode it using a QR generator app e.g. http://tag.microsoft.com/home.aspx or online at http://goqr.me/
- Download your image and stick it wherever you want it used.