The essential words are the key words which we will use in a paragraph summary.
The key words from these sentences, respectively are: Poe, detective (or detective story), two(devices), companion, at the tale's end.
An example summary could be in the following form:
Poe was the inventor of the detective story. He used two devices and it could be found in his story named "The Purloined Letter". The first device is about a companion who tells the story, but does not solve the puzzle. However, the second device shows how the detective discloses the mystery at the end of the tale.