This is a long, mostly well written book on an attempt at 1966 by various international diplomats to broker direct talks between the US and Vietnam. He covers the issues with a great degree of depth and its a fair history of the events in question. Its command of sources and depth of citations are both very good.
But there is a problem. The author and the book stray away from the history of the events into wishful thinking about their meaning and speculation about what could have been accomplished.
The facts are that "Marigold" could, at best, have led simply to direct talks a couple years earlier between the US and Vietnam. But direct talks don't imply any particular outcome. And the idea that direct talks in 1966 would have led to peace when they didn't in 1968 is probably wishful thinking.
The position of North Vietnam was always clear. The only peace they were interested in was a "neutral" coliation government in the south, the withdrawal of the US and the continued presence of their army in the south. That sort of exit strategy was always available and it was available long before 1966. Its not even certain that reaching such an agreement would have had to have involved North Vietnam.
At the end of the book, the author lets loose with all sorts of speculative history. There would have been domestic peace. Johnson would have been easily re-elected with Robert Kennedy following on as President in 1972 with everyone marching into a Great Society Utopia. His thinking contains some obvious mistakes. The strategy of focusing on Beijing was not available to Johnson because China was caught up in the xenophobic chaos of the cultural revolution. Nixon was eventually successful with China only because China drove itself to the brink of war with the Soviet Union. He also (like many) holds on to the false belief that Vietnam was the only source of political turmoil in the late 1960s. But its quite possible that the breakdown between liberals and radicals at the end of the Johnson administration might still have happened but only in a different way over different issues.
The history of Marigold is both interesting and important. But the excessive historical speculation and tendency to overrate where such talks could have led undermines a generally good book.