what is BookCrossing?

BookCrossing is the practice of leaving a book in a public place to be picked up and read by others.
BookCrossing is defined as "the practice of leaving a book in a public place to be picked up and read by others, who then do likewise". The term is derived from bookcrossing.com, a free online book club which began and encouraged the practice. Bookcrossing.com's stated goal is to "make the whole world a library".