What is the significance of the letters "AB" in the book, Fairy Tale: A Novel?

The letters AB are symbols of safety. Bowditch, Leah, and Claudia all warn Charlie that he will remain safe and not become lost in Lilimar as long as he follows the initials that Bowditch has left. Charlie finds himself in a difficult spot when Peterkin discovers Charlie is following these marks and erases them, leaving Charlie lost.