So I saw this recently and knew exactly what it was. Its a tiny library! Take a book, trade a book! They're popping up all over and I decided to trade out books. I stopped, threw the car in park, emergency lights, ran over, tossed my book in and it was easy. There was only one adult book so i went ahead and took it. It's Patricia Cornwell. It'll be a fast read one day. I now know of one on McLendon and another in Kirkwood. Maybe those will be better.
Go to www.littlefreelibrary.org and somewhere in there you can find a map of all the registers tiny libraries!