Also, while reading authors such as Edgar Allan Poe and Herman Melville, I came across some grammatically dubious sentences which negate an already negated word. For example -- "It was not true that the picture was not uncommon." Is this proper grammar? If not, was it acceptable at the time period of these writers?
A."Who" is the correct word to use for the subject of a sentence.
Who are you?
Who can do this?
Who wants to go to town?
"Whom" is the correct form for an object.
To whom should I address this?
You can love whomever you want.
Here is a link that will tell a little more about whom and whom:
http://hometown.aol.com/drcarlperrin/who.html
The example you gave of a double negative is grammatically correct. It is a deliberate use of a double negative to create a positive statement. That type of double negative often seems to me to be a little wordy and awkward. You could more easily say "The picture was uncommon." However, the use of the double negative may give a particular emphasis that the writer desires.
A double negative is only incorrect if the intent to make a negative statement.
Incorrect: I didn't do nothing. (Meaning, I didn't do anything.)
Correct: She was not unattractive. (Meaning, She was attractive.)