the first novel was released in february 2003 under ascii media works' (formerly mediaworks) dengeki bunko imprint, twenty-two novels have so far been released. the novels were adapted into a sixteen episode anime television series directed by takahiro omori and produced by brain's base and aniplex.</onlyinclude> the first thirteen episodes were aired on wowow from july 26, 2007, to november 1, 2007; the final three were released direct-to-dvd. the series was also adapted into a two-volume manga, an adventure video game for the nintendo ds and two drama cds. an additional novel was released with the first drama cd and two "gaiden" novels were released in parts with the dvds of the anime adaption.