There were also two additional volumes published, the first containing an overview of the series and interviews, and the second featuring an illustration collection. Also listed are short stories published in Shueisha's Cobalt magazine, which were included in later volumes of the novels.
In addition to the main volumes, supplementary materials were released, including a first volume that offered a broad overview and insights through interviews, and a second volume featuring a visual representation. Moreover, short stories from Shueisha's Cobalt magazine were also included in later editions of the novels.