It took until about the middle of the book for me to realize how the title fit into the story. Ghost fires are the souls of the dead who have not found peace and they wander. Though Theo and Connie are still alive, they are also like the ghost fires, trying find peace and a happy life and to make the best out of the troubles they find themselves in during the tumultuous times when the British and French are fighting for world domination. They had also lost so many who died too soon and were separated from each other. It was a pretty cool tie in with the title.
I much preferred Theo's parts of the story to Constance's. He was thrown into situations but then tried to make the best of things and find a happy ending. Connie on the other hand was too bold and willfully made the same choices that landed her in some trouble. And she did not seem to learn from her experiences, choosing instead to go after riches and the high life. Theo was more willing to find the good in any situation and I believe he had the happier ending.