Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
The whole point of a good mystery story is to make you wonder, oh hey, is this detail important? Or is that? That four thousand francs has to be a clue – no? Really? OK. And what better way to encourage that kind of constant curiosity than to model it in the reactions of the narrator? This narrator is constantly saying things like, "Tell me, for Heaven's sake, the method" (16) and "you astonish me" (18) and "How was it possible?" (97). The narrator's constant commentary on Dupin's deductive crazy shenanigans keeps up the reader's interest. After all, since the narrator is so intrigued by the details Dupin lays out, maybe we should be, too. The curious mood of the story maintains the freshness of the central mystery as it continues across numerous pages. After all, there isn't much emotion involved in all of this, so Poe has to find something else to draw in the reader, and that something else is the overall tone of intellectual interest and suspense.