Voss builds the book against the classic interest-based school of negotiation: he argues people do not decide by calculation but by emotion, and that the job is to understand the other side's state before proposing anything. From there come his tools: repeating the last few words back, naming what the other person is feeling, and asking questions that force an explanation rather than a yes.
What I took from it
The accusation audit, which is what I use most. Before a hard conversation — a scope that drifted, a date that will not hold — you say the other side's objections out loud before they do. Said by you they lose their charge, and the meeting starts at the problem instead of starting at the defence.
The difference between someone saying you are right and someone saying that is exactly what happens. The first closes the conversation out of politeness; the second is the signal that the other person felt understood, and it is the only thing that later holds a commitment together.
And the questions that start with how, which hand the constraint back without saying no. Asking how something is supposed to be done within the time available forces the other side to look at the constraint rather than argue with it.
What I left out
The haggling. The book offers a staged method for squeezing towards a number, and in professional services that does not transfer: a price that drops because the client pushed teaches them the first one was padded. I would rather change the scope than the number.
And the wrapping. The hostage stories sell the book and distort what it teaches: a commercial negotiation where both sides keep working together afterwards has nothing in common with one where you will never see the other person again. Read as a set of techniques it works; read as an epic, it invites you to treat a client as an opponent.