The only thing which she keeps in her pocket is the whistle which can be used to summon help at the time of need so she thinks that it will come in handy as well. An evil force has been unleashed in the world and its darkness will soon eat up everything in its path if it is not stopped in time.
In the middle, there is a great history linked to that darkness which is narrated by Tim Curry i.e the narrator of the novel. To Hold the Bridge and Sabriel have good stories but don’t prefer those books over this one. This is a much better story in terms of action and emotions. Also, the personality of the character evolves much better.