I have just finished the final chapter of The Invisible World, and this one is going on the list of books I didn't intend to turn out like that. I actually did want there to be more of Jack fighting ghosts and ghouls, but then Brian got clocked on the head with a door. A lot of things were planned, such as Katie showing up out of nowhere and the revelation that it is all just a book, but I think I made the twists work. My main regret now that I've written the last chapter is that I never wrote a chapter or even a scene where Katie visits Jack while he's writing the newspaper article. It would create a bridge between them gathering the interviews at the beginning and the completed story being finished at the end, and Katie would be able to witness him writing down all the names on the list, bringing back all those people even sooner than before. It also ruined every book I will write in the future because I'll want to get meta with it and not be able to because I already used that twist. I even thought of making Brad say "It's the lore chapter!" in a future chapter of Ghostkillers 3, but that's not the kind of book that is.
Just like Jenny mentioned, there is a sequel in the works. It's called The Mind's Eye. Currently, it's behind a novel called Ghoul, which is also a while from now, so please hang in there.