Its a classic series that only has like 2 of the books translated into English from an online community of fans. I've never seen the orginal novels in my hands though I have had other similar titles in my hands. Such series were Teito Monogatari, which actually inspired Street Fighter, PS2's Digital Devil Saga, Suehiro Maruo, and many other pop culture stories and characters in Japan.
The series is known in North America as Tokyo The Last Megapolypse. The novels have 10 volumes with another facebook community advocating for the series to be translated and sold in England and America.
Both Digital Devil Saga and Teito Monogatari share similar themes of other world demons and devils trying to enter and gain control of the world though human creation.
If you're a fan of Digital Devil Saga than I suggest you also look into Teito Monogatari.
You can also buy both Digital Devil Saga's and Teito Monogatari's original 1st edition books on Amazon Japan. If you're really going to get into the series and buy the books than you need to learn Japanese, else its needless materialism on your otaku obsession.