book ‧ cross ‧ ing /ˈbʊkˌkrɒsɪŋ $ -ˌkrɒːs-/ BrE AmE noun [uncountable]
the practice of leaving books in public places or on buses, trains etc, so that other people will read them and then leave them for others to read. There is a website where people can record details of the books and the places where they found or left them.
—bookcrosser noun [countable]