The archangels can be really cool characters in fanfiction. They have their own hierarchies and powers. A fanfiction could explore their politics, their relationships with each other, and how they view the world of humans, angels, and demons.
There are fanfictions that rewrite certain plot points in the Angel Sanctuary universe. Maybe a story where the different factions of angels and demons interact in a more peaceful way than in the original. This could lead to new alliances and romances between characters that were previously enemies.
Sanctuary can also be shown as a community. In some novels, a group of people come together to form a sanctuary. They support each other, share resources, and create their own rules to keep the peace. This shows the social aspect of a sanctuary.
I don't know who the author of 'the sanctuary novel' is. There are so many novels out there and without more information, it's difficult to determine the author.
I'm not sure specifically as there could be many novels with that name. It might be about a place of refuge or sanctuary, perhaps a physical location like a hidden garden or an old building that holds secrets and stories.