From what point of view is "My Last Duchess" told, and is it about Browning's late wife?

It is told in the 3rd person, with a limited narrator.  It is the duke himself speaking, and that is the only perspective we get.  It is just a monologue of his thoughts and perspectives.  So, his guest's thoughts are never portrayed, and we have to infer what the guest is thinking based on what the duke says.  For example, at the end, we can guess that the guest was attempting to leave when the duke states, "Nay, we'll go together down, sir,".  He wants his guest to wait, to not be so anxious to leave.  Because of this limited perspective, it gives us a really great glimpse into the psyche of the duke, but very little into the actual reality or facts of what he is speaking about.  We don't know for sure if his first wife was too flirtatious or unfaithful as the duke implies, because it is just his perceptions.

There is no evidence to suggest that the poem is about Robert Browning's first wife; rather, it is speaking of the narrator's first wife,the man that Browning creates to tell the story.  Browning had a knack for creating creepy characters who go to great lengths to secure love, and the duke is a great example of that.