Find a book

Book data: openBD, NDL Search (via its API), Open Library

Places: Geolonia address data, GeoNames (CC BY 4.0)

Make QR stickers, print them, and put one on the back of a book you've finished.
Scan the QR code and register the book (scan its barcode or search by title).
Hand it to someone, sell it to a used bookstore, or leave it on a shared shelf.
When the next reader records it, the book sends you a letter.