Memory of Books
Who read this book, and how.
Put a QR sticker on the back of a book. When you're done with it, hand it to someone, sell it to a used bookstore, or leave it somewhere.
When the next person scans the QR code and takes a photo, one line is added to the book's timeline: which reader they were, and where. No names. Places only down to the city.
When a book you passed on gets a new record, the book sends you a letter.