For the entire time that I was reading this book, I was wondering what it was about. It's about Vietnam, that much I can figure out. But at least while I was reading the book, I could not have told you what else was going on.
Of course there is a central plot line, and a group of main characters. The book is written year by year, following the different characters as their paths intersect. Two of the characters seem almost completely disconnected from the rest, in that their actions really have no bearing on what happens to the rest of the cast. These are two brothers, ordinary soldiers in the war, who have various troubles with authority. They both end up back in the United States, and in jail eventually. While they happen to be present for some of the plot involving the other characters, once they are in the US the point of their stories becomes less clear. The rest of the characters are all connected by the colonel, Francis Sands, and his attempts at using a double agent without CIA authorization in Vietnam. The colonel is the unknowable hero of the story. The other characters idolize, love, and fear him, in turns. He, and his nephew Skip, are the glue that holds the plot line together.
In the end, the book is about redemption, and what it means to atone for your sins, perceived or real. This doesn't become fully clear until the end of the book, when you realize that the "plot" wasn't what was really important. Tree of Smoke requires re-reading, to fully grasp what the author is trying to get across. There is so much depth, and so many layers, that are truly impossible to get at in the first reading, to do the book justice, it (or at least parts of it) should be read again and again.