How does the poem "The Raven" demonstrate the theme of mystery?

First, who is "rapping" at the narrator's door (21)?  Is it Lenore's ghost?  If so, why has she returned?  If not, who then?  If it were the raven, then he, presumably, would have entered when the narrator opened the door, just as he did when the narrator opened the window.  Did the narrator imagine it?  Is he reliable?  

Then, what's up with this raven?  Why does he rap at the narrator's window?  Why does...

First, who is "rapping" at the narrator's door (21)?  Is it Lenore's ghost?  If so, why has she returned?  If not, who then?  If it were the raven, then he, presumably, would have entered when the narrator opened the door, just as he did when the narrator opened the window.  Did the narrator imagine it?  Is he reliable?  

Then, what's up with this raven?  Why does he rap at the narrator's window?  Why does he conduct himself with "mien of lord or lady" (40)?  Why does he keep saying "'Nevermore'" (48)?  Is it the only word he knows and so he keeps repeating (as the narrator supposes at first) or is it meaningful in a new way each time he says it?  Is he a "'Prophet,'" as the narrator suggests (85)?  If so, is he good or bad -- from God or the Devil?  

We, ultimately, do not find out the answers to any of these mysteries, and we can only use the text to speculate.  The ambiguous interpretations of these mysteries, perhaps, is one of the reasons this poem is so disturbing and fascinating at the same time.