What is the difference between "truth" and "evidence," as the narrator tries to explain?

"Evidence is not truth. It is only evident" (p.30).
A constant theme of In The Lake of The Woods is the difference between evidence and truth. However, this challenges how we usually think of the relation between the two. Surely evidence is needed to establish the truth? However, the standpoint of John Wade is different and is entirely in keeping with the air of mystery and lack of closure which pervades the whole book.
Many of the chapters are entitled "Evidence." Here, Tim O'Brien sets out, often in painstaking detail, pieces of evidence relating not just to Kathy's disappearance, but also to John's alleged involvement with the infamous massacre at My Lai during the Vietnam War. The author is setting before us an exhaustive, comprehensive collection of facts which we as readers are required to put together. It is not up to the author to present the truth, hints O'Brien; he/she can simply put together certain fragments and perspectives and allow readers to make up their own mind.
O'Brien's approach speaks to what he sees as the virtual impossibility of establishing the truth in any meaningful sense of the word. To that end, he is advancing a typically postmodern understanding of the truth, one that displays suspicion of any overarching meta-narratives that somehow transcend the complex world of reality and purport to establish the truth from an objective perspective.
The open-ended nature of claims about truth shown throughout the book is also suggestive of the philosophical distinction between fact and value. O'Brien piles up fact after fact, without at any time establishing the truth of the matter. The compilation and collation of facts requires a detached perspective, a way of looking at things associated most readily with the approach of scientists. The scientific approach gives us the factual evidence, but not the truth. The truth, if it is ever to be established, is something that must be lived and something that must be experienced by living, breathing human beings thrown headlong into a particular social and historical context.
As readers, we are forced to attempt to find the truth, if we wish to, by a process of empathetic identification with the book's characters. We must put ourselves in their shoes, retrace their steps, and live their lives in a spirit of imaginative reconstruction. We may never achieve anything by way of a conclusive ending (indeed, O'Brien would deny the possibility of ever doing so), but at least in making the attempt we would surely come to a deeper, more adequate understanding of how our notions of truth are often constructed in the here and now, rather than being handed down to us from a transcendent beyond.