In re-listening to Grave Peril, I latched onto Lydia's prophecy that she told Harry near the beginning of the book. The prophecy fits with the story of the book, but also seems to set up the entire series. It's pretty cool...
"Fire," she whispered. "Wind. I see dark things and a dark war. I see my death coming for me, out of the spirit world. And I see you at the middle of it all. You're the beginning, the end of it. You're the one who can make the path go different ways."
The last one, about Harry being the beginning and end of it and he can make the path go different ways... hell, it feels like it could refer to Mirror Mirror where Jim has said that the premise is that Mirror Harry made a different choice in Grave Peril...