The system the book popularises sorts behaviour into four types — dominant, expressive, steady and analytical — and descends from a 1920s model that later became a commercial product. The foreword to this edition is signed by the heir to the company selling that assessment, and the book never examines its validity: it treats wide use as proof, which is not the same thing.
What survived
One idea, and it is not his: the same information has to be delivered differently depending on who is listening. Some people want the conclusion in the first line and lose patience with the reasoning; others will not accept the conclusion without seeing the evidence under it. When I prepare a presentation I think about those two people separately, because they are almost always both in the room.
And a useful side effect in reverse: reading the descriptions exposes your own bias when explaining. You explain the way you would like things explained to you, and that works on about half the room.
What I left out
The rest, starting with the premise. Labelling someone with a colour is the fastest way to stop listening to them: once I have decided what type someone is, everything they do reads as confirmation. In a discovery meeting that is exactly the opposite of what is needed.
And the title, which is not a detail. A book that offers to help you understand others and calls them idiots on the cover is selling something else: permission to believe the problem is always on the other side. I read it through and it is on this list for the sake of honesty, not as a recommendation.