I saw this on our local news this week. I had seen one of these book boxes on our bike ride route and assumed the home owners set it up on their own for their neighbors. But this is a "little library". Really cool.
"Little Free Library is a nonprofit organization that supports the worldwide movement to offer free books housed in small containers to members of the local community. The organization is based in Hudson, Wisconsin, US. Little Free Libraries are also referred to as community book exchanges, neighborhood book exchanges, book trading posts, pop-up libraries, and micro-libraries,  and have been likened to human bird feeders.
The idea was popularized in Hudson, Wisconsin, in 2009 when Todd Bol mounted a wooden container designed to look like a school house on a post on his lawn as a tribute to his mother, who was a book lover and school teacher. Bol shared his idea with his partner, Rick Brooks, who spread the word, and the idea spread rapidly. Library owners can create their own library box, usually about the size of a doll house, or purchase one from the website. For a fee, libraries may be registered and assigned a number at the organization's website. Owners receive a sign that reads "Little Free Library". They often have the phrase, "Take a Book. Leave a Book." The Little Free Library Index lists locations with GPS coordinates and information.
The original goal was the creation of 2,150 Little Libraries, which would surpass the number of libraries founded by Andrew Carnegie. As of January 2014[update], there are over 15,000 Little Libraries worldwide. As of February 2013[update], all 50 U.S. states and 40 countries worldwide have been involved in the program. An estimated 1,650,000 books were donated and borrowed from 2010 to 2013."
Here is the link to the organization that helps you set one up in your yard. Hmmmm....