This is one of the few books that deals with the human element in security (mainly IT security here), known as Social Engineering (SE, for short). It begins with a short example-driven overview of the technical aspects of hacking humans (Open Source Intelligence, OSINT), but it is clear that this is not where Hadnagy's heart lies. Rather, he likes to talk/write about the aspects of SE that involve a direct interaction with humans, and about 3/4 of the book are devoted to these.
One should note that the subtitle "Science of Human Hacking" is not really appropriate, as the practice of human hacking is not science. Some scientific results are quoted in this book, mainly from psychology, but they serve to give names, categories and structure to the techniques and concepts that social engineers have been using before anyway. None of the social engineering techniques is actually developed through the application of science or by a scientific process (it would be perfectly fine to call SE an art, as in the first edition of this book).
Also, you will not learn how to be a social engineer from this book. Pulling off the kind of deceptions and manipulations necessary for this work requires some very particular personality traits, brazenness or chutzpa if you like, and very strong nerves. Maybe some of this could be learned, but not from reading a book. Nevertheless, the information in this book is valuable for anyone wishing to understand SE better, and can be useful, say, for integrating SE into a Red Team exercise or to increase security awareness in the employees of a potential target.
The text proceeds by chapter-wise addressing certain manipulation techniques and exploitations of human behaviours and feelings. It is heavily based on anecdotes to illustrate its points. Not all of these examples fit the point they are supposed to make very well, though. In some places it feels like the Hadnagy simply wants to tell that particular story and then comes up with a forced reason why it should illustrate a certain point. Anyway, these anecdotes are entertaining and at least teach you something, even if it is not always what they are meant to teach. Some reviewers wrote that this is an "American-style book", meaning it is verbose, meandering and boastful. That is true. The book is clearly aimed at an American audience, and I would really like to see some of Hadnagy's more charm-driven hacking approaches tried against people from a different culture, say Germans or Russians. As an aside, there is actually an interesting aspect to this, which is not addressed in this book but in an episode of Hadnagy's podcast (the one on "baking a human cake"). A guest on this episode explains that human hacking essentially works by manipulating some basic feelings, which is independent of one's culture, but the way to go about it can depend strongly on the cultural background of your target. This might have been a good addition to the book.
Another thing I found bothersome was that throughout the book, Hadnagy tries to frame SE as if the social engineer was doing something for the benefit of the target person ("make them feel better for having met you"). The idea is that you employ manipulation techniques that play on the positive feelings of your target person rather than their negative ones. Make no mistake! A social engineer is essentially a con-artist, even if he/she gets hired to do a penetration test that is ultimately for the benefit of the customer. But regardless of how you frame it, what you do is lying and manipulating people. Making a person feel better for having met you is not the same as actually making the person better. SE gets you what you want, not what your target person wants. And even if it happens in the bounds of a penetration test that helps a company improve security, the person who ultimately fell victim to the social engineer will not feel better about having been tricked, and will possibly suffer other consequences as well. The idea behind this mantra of doing good by SE is simply for social engineers to be more at peace with their work.
For a 3rd edition of this book, I would wish for the text to be more streamlined, less boastful and better structured, SE to be called an art and not a science, a take on cultural aspects of SE, and a bibliography with references for the quoted scientific works and for more in-depth exploration of some topics.