Here's how it works. You and your students register a book that has been read to receive a Book Crossing Identification Number (BCIN), and you attach a label with instructions as provided for free by the site on a label or bookplate and leave the book for someone else to find. They then register the book as found, read the book and repeat the process. The students will be able to see where their book has traveled too.
I plan to use one of our class read alouds to register and as I travel to my sisters wedding in another state, I am going to leave the book in an airport for someone else to find.
I think it is a great way to get students to share their love of reading with other children around the world.
Also, click here for a great article Introducing Bookcrossing in the Classroom.