poe wants to explain the writer's process to an outside observer. selecting nonfiction, specifically the expository essay, is appropriate because expository essays examine a topic in an organized way. the expository essay makes use of logical examples and explanations to support an idea, which is well suited to poe's purpose. whereas king wants to explain how he became a "good writer" (king's words) to a fellow writer. selecting nonfiction, specifically the memoir, is appropriate because memoirs recount personal experiences and development, the exact ideas addressed by king. the memoir makes use of narrative techniques, which would allow an author who is explaining how he became a "good writer" to use evidence from his life easily.
most likely the most credible source would be the diary of anne frank.