Bookcrossing is founded on the idea of registering a book, then leaving it somewhere in public for someone else to find and read. The group now has 400,000 participants in 120 countries. “The founders of BookCrossing.com compare their online book club to a virus, one that has reached far-flung places carried by members who heed the philosophy: if you love a book, set it free. One selling point is that it costs nothing to join. Members include literature buffs determined to share their passion or thin out their shelves and travelers who simply love a good book — although here the books do most of the traveling.”