No matter what kind of book you are writing, you still have to do research. I did a lot of reading up on world mythology, especially anything having to do with good and evil spirits. I wanted to make sure multiple types of demonic myth were present, not just the Christian view of them, so you‘ll find Japanese, Indian, Tibetan, and other kinds of demons represented (plus the kind I’ve made up.) I read a lot of old “demonologies” — there was a whole time period where scholars were obsessed with listing every kind of demon and mapping Hell. By the same token, just because I live in New York didn’t mean I didn’t have to do research about the locations I wanted to use. I traveled around the city taking photos for reference and read up on interesting/mysterious places in the NYC area. For instance the smallpox hospital Valentine is using as a hideout in Book One actually exists, but I had to visit it and research its history before I could use it in the book.