Ishibumi and Miyama-Zero launched the High School DxD novel series in 2008, and ended it with the 25th volume in March 2018.
The pair then launched the Shin High School DxD novel series in July of that same year, and Kadokawa published the third volume on August 20.
Ishibumi actually wrote the first volume of the story in 2006, before his more popular High School DxD series, and the novel was then titled Slash/Dog.
That series only received one volume, before Ishibumi revived the series with a new first volume in November 2017, with the full title of Daten no Inugami -Slashdog- High School DxD Universe.