Well, there are prequel novels that offer background on some of the characters and events. Also, there are companion novels that explore additional adventures and story arcs within the same world.
There aren't many direct novels specifically tied to High School DxD Season 3. But some spin-off or companion novels might exist that expand on the story or characters.
The main themes in High School DxD novels are power struggles. Different groups, such as the devils, angels, and fallen angels, are vying for power. Romance is also a big theme. Issei has his harem of girls, and his relationships with them are an important part of the story. And there's the theme of self - discovery. Issei is constantly learning about his own abilities and his place in this supernatural world.
Well, in addition to Issei, there's Asia Argento. She's a nun - turned - devil and Issei's friend and love interest. Kiba Yuuto, a devil swordsman, is important. He has his own backstory and plays a significant role in battles. And Xenovia, a former exorcist turned devil, is also an important character.
It's hard to give an exact number. The series has been ongoing for some time, and new volumes may be released. You could search on dedicated book sites or ask in related online communities for the latest count.
In a Naruto and High School DxD fanfiction, there could be a scenario where the protagonists of both stories meet in a neutral dimension. Issei from High School DxD might be intrigued by Naruto's ninja skills and determination. Naruto, in turn, could be curious about Issei's Sacred Gear. They could start training together, sharing their knowledge and skills, and ultimately have to face a common enemy that threatens both their worlds.
One popular one is 'High School DxD'. It's known for its unique blend of supernatural elements and high school setting. The story follows Issei Hyoudou, a perverted high school student who gets killed and then revived as a devil. It has interesting characters and a lot of action-packed plots.