A substantial organization has grown up around the Little Free Library concept. Its website has tips, plans for making libraries, kits, even T-shirts. You can view a map of Little Free Libraries around the world at http://bit.ly/LittleFreeLibraries.
Another, larger book-sharing method is BookCrossing.com, begun in 2001. Remember the dollar-bill-tracking sites where you typed in the serial number of a marked bill to see where it had been? This is a similar deal, only with books. You register a book on the site to get a tracking number, which you write in the book before giving it to someone or leaving it somewhere. Whoever picks it up is invited to use the tracking number and website to let you know where it has gone. There are 2 million BookCrossers in 132 countries and 10 million registered books.