It's the newest branch of the Little Free Library, a concept that began with a single mini library in Wisconsin in 2009 and has since spread to over 15,000 locations around the world.
Passersby are welcome to stop by and take a book, then return it when they are done. They may also contribute a book of their own. The small collection depends on the honor system; no money is exchanged, and no library cards are required.
According to Little Free Library's website, there are two other outposts in San Francisco – one in the Outer Mission, and one in Sea Cliff. The Duboce Triangle version will be located near 221 Noe Street, between 15th and Market (home of the ever-changing Noe So Cute lemonade stand).
Tomorrow (Sunday) at 11am, the library will make its debut with a ribbon-cutting ceremony. There will be free coffee for adults and balloon animals for the kids. If you attend, just be sure to bring a book to help get the collection started.