3.5/5.
This book was very difficult to rate. On one hand, it's a pretty comprehensive guide of human gestures and postures, aided in their understanding by suggestive illustrations. On the other hand, I kept getting the feeling that the authors were trying really, really hard to sell me a commercial version of body language. Often, things were exaggeratedly emphasized in a clickbaity way and I would not have been surprised to see chapter titles such as: "You will not believe how your boss is dominating you!" or "Seven ultra-secret signals she's into you!".
Searching for information about the authors, I see that neither of them is a psychologist or scientist of any kind. They're just really, really good salespeople who've found their niche. That makes me a bit skeptical about the validity of the interpretations they offer.
There are other clues that make me doubt the content. One is the studies cited at the end. The book often contained phrases such as "studies show" or "research says". However, if I wanted to read about the study mentioned in chapter X, point Y, and flipped the pages to the end of the book, surprise! Instead of showing the research in the order it is cited, it is shown alphabetically. That means that I have to wade through a few hundred cited sources to see which applies and I am pretty sure this was done on purpose. The other objection I have with the studies is that most of them are from the 70s. (The newest ones are from 2003.) That leads me to believe that a) a lot of this research has been debunked/updated/couldn't be replicated and that b) the authors purposefully selected only those sources that had a vague resemblance to the point they were trying to make and stretched the interpretation in a sensationalist manner.
A few other things bothered me. One was the "old gramps who's trying to be hip" cringe humor. The second was the outdated examples and references - it really shows that the book hasn't been edited for a newer audience, but is still stuck somewhere in the 80s-90s (in mentality too). Then there were the problems I had encountered in the previous body language book I had read, although to a lesser extent here.
First, the book had a really strong emphasis on man vs man or man vs woman relationships.
Second, there were very few references to woman vs woman relationships. (In fact, the authors presented a very petty/catty attitude between women as the only plausible one. What about female friends? Is the notion that alien?)
Third, all the interactions between men and women were interpreted in a sexual manner. (What about the young man arguing with the old woman in the bus over the position of the window? I bet the authors would say that the opening and closing of the window is a suggestive sexual allusion or whatever.) More so, women were seen as always submissive in their gestures in a man's presence. (I'm not going to go into that.)
Fourth, men were also presented as some sort of primitive grunting beasts ready to jump at each other's throats... OK, I am exaggerating, but the authors did seem to see them as very simplistic beings incapable of any kind of complex thoughts. (I find this particularly insulting.)
Again, I think that most of the things mentioned in the paragraph above have a very high commercial value and while they might carry some form of truth (yes, there are differences between the two sexes), they have been exaggerated for the public, because this is what sells and this is what the public wants to hear/read.
All in all, this was another book I used as research for the series I am writing. The best thing about it is that a) it's comprehensive (there are even chapters about mirroring another person's body language or height differences or seating positions) and b) it has useful illustrations. I recommend it to other (aspiring) writers, just be careful to take it with a grain of salt.
(As a bonus, checking this book, I realize that my characters' body language has been pretty well-written, just too detailed and hard to follow at points.)