Suppose there were a tool that would allow you to process items, much like you would for spine labels, but instead printed custom messages along with a QR code to useful things.  For example, you might select a "Comment QR label".  You would enter a generic message like "Tell us what you thought of this book."  Labels would print for each item with this message, and the QR code beneath, and you would affix this label inside the back of the book.  When the patron is done reading the book, they scan the code, and it takes them straight to the comment form in the OPAC (after signing in, if required) for that book, and they can fill out a comment right on their device.

Other options, instead of comments, might include rating, tagging, or bragging about the book in a specific social media.  For example, they scan the QR code and it would go to Facebook and start a post with a link to the book in the OPAC.