How we cite our quotes: (Paragraph)
Quote #1
"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)
As we soon learn, it's secret because the royal lady has a secret she doesn't want the royal man (or the public) to know about. By concealing the details of this from the reader, the narrator too is practicing deception—deception by omission.
Quote #2
"During its perusal she was suddenly interrupted by the entrance of the other exalted personage from whom especially it was her wish to conceal it." (28)
In most mysteries, the goal is to reveal a secret that's been concealed. Here, the goal is to keep the royal lady's secret from being revealed, by revealing the secret location of the letter, which was purloined from her. What a tangled web of deceit!
Quote #3
"To be sure, it was to all appearance radically different from the one of which the Prefect had read us so minute a description." (115)
But…but we thought you said it wasn't hidden at all? The point Dupin seems to be making is that even the best disguise leaves a hint of the person (or letter) underneath it. No deceit can be entirely effective.