The main value of "The Tell-Tale Heart" is to be found in Poe's mode of narration. He was one of the earliest users, if not the originator, of first-person narration by an unreliable narrator. This unnamed narrator also happens to be a maniac, although he doesn't realize it and refuses to admit it. Poe's short story is a masterpiece because of the way it is told. The illusion is maintained right to the very end. The description of the narrator's delusion of hearing the beating of the heart of the old man he had murdered and dismembered is so well written that the reader is likely to imagine being able to hear it too. This narrator is insane but able to render accurate descriptions of every detail of his crime.
One of the best sentences in the story is the following:
I turned the latch of his door and opened it--oh so gently!
The dashes after "opened it" prepare the reader for a change of tone with the words "oh so gently." This is unconventional narration. It is more like dialogue than exposition. Every reader can remember opening a door in the same way, oh so gently, so as not to waken someone--but without the intention, of course, of committing a murder. The reader knows that the old man is going to be killed sooner or later. The suspense keeps building until the eighth night.