In my opinion, the general theme of The Lake of the Woods is as follows: investigation leads us nowhere (especially in the realm of war).
We are fascinated, all of us, by the implacable otherness of others. And we wish to penetrate by hypothesis, by daydream, by scientific investigation those leaden walls that encase the human spirit, that define it and guard it and hold it forever inaccessible.
The entire novel is an investigation of the disappearance of Katy Wade. The reader learns a vast amount during the novel! It is an incredible and thorough investigation! We learn about John Wade's Vietnam War service. We learn about his participation in massacres of unsuspecting Vietnamese. We learn about John Wade's youth. We learn about John Wade's courtship and love of Kathy. We learn about John Wade's marriage to Kathy and their life together. We learn about their vacation near the lake of the woods. We learn that Kathy has disappeared. We learn about the court battles that ensue and the suspects named. We learn that the author has given us grand and specific footnotes on anything and everything involved in the investigation. We learn lots of hypotheses about what happened to Kathy. What the reader does NOT learn is what ACTUALLY happened to Kathy Wade! Worse, we learn that the vicious cycle of pointless investigation will begin again when John Disappears at the end of the novel!
My heart tells me to stop right here, to offer quiet benediction and call it the end. But the truth won't allow it. Because there is no end, happy or otherwise. Nothing is fixed, nothing solved. the facts, such as they are, finally spin off into the void of things missing, the inconclusiveness of us.
In conclusion, let me say that it's important to put this theme of nil investigation in the context of the futility of the Vietnam War and all of the needless deaths it caused. By the end of the book, we can conclude that both Kathy and John are two MORE of those needless deaths.