The Book of the Azure Sky (蒼天の書 Sōten no Sho, officially/Ger. Buch der blauer Himmel) is the device type::Storage Device used by Reinforce II, similar to how her own master Hayate Yagami uses the Tome of the Night Sky. Initially, the Book is shared by both Rein and Hayate. After Rein has passed the Administrative Bureau's employment examination in the autumn of 0069, it becomes necessary to increase data capacity. As a result, a division is needed and Hayate has then created her own Tome, letting Rein to use the Book of the Azure Sky exclusively.