"There is no friend as loyal as a book," Ernest Hemingway once wrote. On World Book Day, we are honoring these loyal friends in the German city of Stuttgart, where a huge monolithic cube is both a modern landmark and home to the city library. Opened in 2011, this striking building was designed by architect Eun Young Yi to be a calming, meditative space in a bustling city. Its interior is white, with splashes of color provided by the books that line its walls.