I love Tim O'Brien. I have since I read The Things They Carried. In the Lake of the Woods does include elements of Vietnam, but those elements are merely a backdrop to a larger story about a marriage and a disappearance. While this book isn't nearly the mystery that some marriage trouble/disappearance books are (ex: Gone Girl), it delves into the psychology of one man (John Wade) and how a series of events (including the massacre at My Lai) sent him on a deep, dark path. 
 In quintessential Tim O'Brien nature, he never reveals any "truth," so to speak. If you wonder what I mean by that, read the short story "How To Tell a True War Story," and you'll see O'Brien's obsession with telling/not telling the truth. 
 The book is broken up into various sections-- some flashbacks, some hypothesis, some in present time, and ones labeled "evidence" where the reader gets very small quotes from various sources on a variety of subject. This does increase the difficulty of the text, but if the reader if reflective enough, it all comes together. 
 While this doesn't replace my The Things They Carried love, it was an enjoyable, troubling, and mysterious read.