Which effect does it have on Poe's readers that he doesn't expose his unreliable narrator until late (in the story) and therefore forces the readers to read his stories again and again, to get a grasp on the narrator?
This strategy used by Poe has the effect of making us, the readers, lose our bearings somewhat. We are from the first drawn into the intense world of Poe’s narrator – and as it is a first-person narrative, we can only go by whatever the narrator chooses to tell us, we can only follow what he says. At first, we may be entirely accepting of what he presents to us, but gradually as we realize he has some sort of kink in his personality, we are left unsure of how to react to the story; ultimately (as is generally the case with any unreliable narrator) we might come to doubt the entire veracity of his tale. We are plunged into the world of the narrator but when we realize he is less than trustworthy – indeed, Poe’s narrators are often quite unhinged – we are left floundering. We have to go back and re-read the story to try and get a proper handle on what is going on, trying to work out why the narrator is behaving as he does, and what the events and other characters of the story are really like. We have to try and get behind the emotionally-driven narrative we are presented with. The generally intemperate, overly-emotional and passionate narrator in a Poe story tends to have a marked distorting effect. We have to go back and read between the lines, as it were.
In other words, Poe makes us, as readers, work that little bit harder at trying to figure out just what is happening in the story and why. As we come to distrust the narrator’s version of events, we have to ponder alternatives. For instance, in those stories which include apparently supernatural events, we come to question whether these actually occur or whether they exist solely in the narrator’s fevered imaginings (for example in ‘The Black Cat’ or ‘Ligeia’). Sometimes, crucial facts of a story are left hanging as the narrator refuses to clarify them for our benefit. For instance, in 'The Cask of Amontillado’ Montresor never bothers to explain just what Fortunato did to him to incur his lasting and implacable enmity. We might at first read the story expecting to be told what Fortunato did, but all we get is Montresor’s elaborate scheme for revenge which he executes with fiendish glee. In the end we are left to speculate whether Fortunato really did anything at all, or whether Montresor is purely driven by the need for murder and vengeance.