"The Purloined Letter" is told in the first person, by an unnamed narrator, who doesn't participate directly in any of the story's major action. This narrator is Dupin's roommate, and he also narrates the other two Dupin tales of detection. In those stories ("The Murders in the Rue Morgue" and "The Mystery of Marie Roget"), the narrator provides much more of his voice and opinions than he does here, where he's basically a talking head reporting on other talking heads.
In fact, the narrative structure of "Letter" is unusual, even for Poe. It consists almost entirely of dialogue in three conversations. But, even in the third conversation, which is solely between the narrator and Dupin, the narrator doesn't say much.
Except that he's actually saying everything. In other words, he isn't playing the conversations back like a tape recorder, or whatever kids use these days to record songs off of the radio. (What, people don't do that anymore?) No, he's deciding what to tell us, and what to hold back. For example, he gives us Dupin's full name, but only the initials of D— and G—, (Check out "Names" under "Character Clues" for some thoughts on the matter.)
And it sure would have been nice if he'd given us the actual dialogue instead of a really crummy summary in the following passage:
"Oh yes!"—And here the Prefect, producing a memorandum-book, proceeded to read aloud a minute account of the internal, and especially of the external appearance of the missing document. (73)
Argh! The internal contents! That's what we've been waiting for all along, but too bad for us. As Jacques Lacan points out in his famous seminar on the story:
[A]ll this tells us nothing of the message it conveys. Love letter or conspiratorial letter, letter of betrayal or letter of mission, letter of summons or letter of distress, we are assured of but one thing: the Queen must not bring it to the knowledge of her lord and master. (source)
The narrator deliberately withholds information that he heard with his own ears. So, how much can we really trust him? More importantly, why is he holding things back?
To answer the second question first, we point to the following quote from G—:
"I will tell you in a few words; but, before I begin, let me caution you that this is an affair demanding the greatest secrecy, and that I should most probably lose the position I now hold were it known that I confided it to anyone." (17)
Ah-ha! G— and Dupin allow the narrator to listen to these private, highly confidential stories because they trust him to be discreet. If the narrator told us everything (names, for instance, and contents of letters), he would be showing us that he isn't worthy of their confidence. By keeping secrets, he's trying to convince us that he's telling the truth.
The compromise he reaches is giving us enough information to understand what he thinks is important—which apparently does not include contents of the letter. The narrator is more interested in the movements of the letter, the fact that it's being hidden from the royal man, and the processes of detecting its whereabouts. In other words, we're totally at his mercy here.
And he, meanwhile, is at the mercy of the people who are telling him these stories. He has no access to information except through G— and Dupin, so we're actually twice separated from the action of the mystery.