Bookcrossing is the practice of leaving a book in a public place — a café, a bench, a bus, a train — so a stranger can pick it up, read it, and then leave it for the next person. People sometimes write a note inside, or record the book on a website so you can see where it has travelled. It is a hobby and a small sharing movement, not an old farm word. Informal-to-neutral. You will see it on stickers and in stories about passing books along.