Note: this section rests on my understanding of the film, not on search results.
The book is tied to Sarah Bellows, who was blamed for harming local children and confined by her family. Her stories were a form of revenge, or a call for the truth. The victims' tales are written one after another, and the group's attempts to stop the book by destroying it or returning it fail. Stella realizes that Sarah's story, and the truth about how she was treated, must be told. She confronts the book on its own terms and uses its own method, writing, to end it. The ending is bittersweet. The immediate danger passes, but the lost friends stay lost, and the teens are left with the knowledge of what they witnessed and the wider world's darkness. The film closes on the idea that the story is not entirely over.