At the beginning of the story in Avi's The Man Who Was Poe, Edmund trusted more in adults' abilities to help him, a mere child, than he trusted in his own abilities. Though, at the start, he wasn't sure Edgar Allan Poe, calling himself Mr. Dupin, would truly help him, Edmund believed he needed to rely on the help of some adult, so he chose Mr. Dupin. By the end of the novel, however, Edmund is really the main person who rescues his sister, not the adults around him. Edmund's successful rescue of his sister helps him realize his ability to accomplish his goals, without the help of adults. In addition, Edmund comes to realize that the goals of adults, like the goals of Poe, may not fully align with Edmund's own goals, making any help adults can provide very limited.
Edmund realizes the limited abilities of the help Poe can offer the moment Edmund comes to understand Poe wasn't truly interested in saving Sis's life; rather, he was only interested in writing an interesting story. We see Edmund reach this realization when he suddenly asks Poe the following in the final chapter:
You never did want to save my sister, did you? ... You only wanted to make sure she'd die (Chapter 22).
One reason why Edmund's assessment of Poe's motives is true is because Poe saw such a strong parallel between Edmund's life and his own life. In Poe's own life, Poe's wife, named Victoria but called Sis, died very young. Unable to escape the torment of her loss, Poe can't help but see the death of a Sis, or even all death, as inevitable. In Poe's mind, the only way to make someone live forever is by capturing the person's story on paper, which is what he means when he gives the following reply to Edmund's comment:
I ask you: in what fashion will your sister live longer? In her life? Or, in this, my story that would have been? (Chapter 22)
What was just a story to Poe was Edmund's real life, and unlike Poe, Edmund had a chance to guide the ending of his own story by rescuing his own Sis. To do that, he had to separate himself from Poe and Poe's own goals, which is what Edmund came to realize by the end of the story.