Blackletter is the font of the stylized letter “S” found on the Cover Sleeve (Slipcase) and page S. (Inscribed Image) on page xiii. This font was used around the 11th century in Western Europe. The Nazis used this font.
According to an article from Bleeding Cool, this font was used in one of the first teaser trailers released for S. It allowed for connections to be made back to the book. This article can be read here.