"The Tell-Tale Heart" was first published in the January 1843 issue of The Pioneer (a very short-lived journal edited by James Russell Lowell). The story has become, I think, the quintessential Poe horror story. It is the first work of his that I read, back when I was in seventh grade. The fact that I enjoyed it then shows that it is an accessible, effective, and fun story. The fact that I still enjoy today attests to its depth as a piece of Art; every time I read it or hear it read I find something new in it.
The story pulls you in immediately with its first word: "TRUE!" That one word, which I always interpret as a shout, grabs you and forces you to pay attention. I find the word incredibly profound too. The narrator who speaks that one word is, after all, about to tell the truth (at least, how he sees it), regardless of consequence. The truth is, in fact, that he brutally murdered an old man whom he loved for no reason other than a slight physical malformity in his pale blue eye. And, of course, with that one word readers look forward with whetted appetite for a story told by the bad guy, the evil murderer — the story is not a "whodunnit" like "The Murders in the Rue Morgue" (the world's first detective story): we know who the killer is, and he's about to tell us from his own lips (pen?) the truth about exactly what happened.
His willingness to tell the truth behind his devious and calculated murder, as well as the careful steps taken to conceal the body, are meant to reassure the reader that he is perfectly sane. Someone who was crazy, perhaps, might try to cover up their role in the murder or suppress some details. Not so with our tell-tale narrator. He willingly admits that he is a cold-blooded murderer, further proven, he thinks, by his careful attention to detail in performing the deed - and what he purports to be his rational state of mind throughout the tale is, in fact, what makes us uncomfortable enough to question his sanity. The term "tell-tale" in the title emphasizes this truth-telling, as well as adding the pun that the "heart" at the end of the story is what is most telling.
Of course, the old man's heart could not still be beating beneath the floorboards. The man was long dead at that point, chopped to pieces, his heart long since stopped. We assume, then, that the heartbeat isn't real and only in the narrator's head. This hallucination, perhaps a manifestation of the guilt he feels for performing his gruesome deed, clearly shows that he is not in his right mind.
But, what if?
One of the things that makes this story (and so many of Poe's stories) so great is that it just might be possible to believe what is presented on the surface; everything can be held perfectly accountable. What if the narrator is perfectly sane? What if he really does have an overacuteness of the senses, as he claims? What if he can really hear all things in heaven and Earth (and many things in Hell)? Should we assume that the old man's heart is really beating in heaven (or, perhaps, Hell)? In that case, there's nothing irrational about the narrator at all.
Another scholar has suggested another option for what the narrator hears at the end of the tale. In the middle of the story, our supposedly-sane narrator takes a moment to listen to the death-watches in the wall. He might be referring to a death watch beetle, an insect which is known to knock its head against solid surfaces, making a rapping sound not unlike a heartbeat. If we believe the narrator has particularly good hearing, it's possible that he really did hear a beating — not of a hideous heart, but of a tiny little bug. His guilt, then, manifests not a hallucination, but a paranoid belief that what he really hears is actually something more disturbing.
We could never prove that Poe was writing a story which subtly featured an obscure beetle at its epicenter — and that is certainly not my intent. I use "The Tell-Tale Heart" as an example of why Poe is such a good writer. On the surface, it is a great horror story, which captures us with its first word and still chills both children and adults with its climactic final line, "It is the beating of his hideous heart!" Beyond that, it is a story just as calculated as the fictional murder it describes, with every element carefully chosen to take the reader in several directions (some of which he/she may not be fully aware). It endures for me because I can go back and consciously follow any number of these paths, or allow the story to take me wherever it may.