Poe's greatest success with "The Facts in the Case of M. Valdemar" is that he managed to convince the world that it was a true story. It's about a man who undergoes hypnosis at the very moment of death, prompting countless letters of people demanding to know more about what they believed was a scientific experiment.
Why was Poe's story so well-received as truth? As the title indicates, the facts of M. Valdemar's condition are laid down in a very matter-of-fact manner, allowing us to trust the narrator's observations of his patient who buoys between the realms of death and life in terrible detail. Though there may be too much detail if you're weak stomached, as Poe enumerates every oozing orifice explicitly and unflinchingly.
It's Poe's unflinching attention to the particulars of "the case" that prompted Victorian poetess Elizabeth Barrett Browning to give props to Poe for "making horrible improbabilities seem near and familiar."