I did In The Lake of the Woods. I think it's a good novel, but quite confusing to try and study. If you've read it, you'll know what I mean. If just seems like an endless mystery. Most people have their opinions about what happened, but you don't get a full story. If you want to make it an exam text, it'll take a bit of dedication.
I recommend learning about postmodernism. This is probably the ultimate English book, because you can easily have a range of different viewpoints and reasonably defend them. In a way, this is very much the point. I think that to write an 'exceptional' essay would involve showing an understanding of this.
Concentrating also on some of the imagery the author uses is a good idea too as there are lots of vivid scenes and interesting writing (I remember the author talking about a red moon whilst John was in Vietnam, which is very biblical; and of course, the two snakes eating each other up).
Think also about how much you blame John for the events in the novel. If you think that he's a product of his childhood, then perhaps look into some Freudian psycho-analysis (it's not as difficult as it sounds, just wiki it or whatever), but also think about how you would feel if John wasn't the protagonist. For instance, if the story was reduced to a short newspaper article. Why would the author have you feel sorry for someone who we know committed heinous war-time atrocities (if, indeed, you feel sorry for him at all)?
I think the theme of redemption and blame is pretty important. Can John be forgiven for his involvement in the war because of how much he suffered afterward? Can he be forgiven for putting his political career ahead of everything else -including Kathy - when you consider his childhood, where he suffered from an unfulfilled requirement of acceptance? If he is responsible for Kathy's disappearance, can he be forgiven, given that he loved her and suffered without her? How much is John free from the restrictions he had in his life and responsible for his actions? Can we at once feel empathy and dislike the things he did? Does he deserve to suffer?