The Flinch is a self-improvement book that has a very focussed attitude; such is the necessity of a free, succinct Kindle edition. It focuses on "The Flinch", what I interpret the author to believe is the natural human instinct of fear and lack of confidence, in order to protect ourselves from harm. The author asserts that this prevents us from reaching our full potential.
I shall initially say that the author comes across as very brutish, almost intimidating with his writing style. The manner in which the book is written seems to suggest that you are worthless and weak because of this natural (note, we can't help instinctive biological instincts) human reaction. As a result, I came out of this book feeling somewhat uncomfortable with what it had to say; it's not written in a very warming tone.
I'd also suggest that the book is too prescriptive in its treatment of "The Flinch". Whilst avoiding such "flinch-like reactions" may have its uses, I found myself wondering throughout the book, "Well, if everyone behaved with this bull in a china shop type manner, it wouldn't be a very good place to be". He cites an example of someone drinking a morning coffee, what I see to be as a metaphor for comfort. He states you should be challenging this comfort zone. But why should you? If you're happy and content with life, why go on to tell yourself that you want out?
On a positive note, I can certainly take positives from the book. I'd certainly recommend reading it at least. His experiments are good (although even now, breaking a mug still seems pointless to me) and you can gain a lot from them. Using "The Flinch" as a tool for dealing with minor issues, such as confidence with strangers, or starting a new hobby is very good. However such a brutal and almost thoughtless attitude towards more sensitive issues such as relationships and jobs may result in more pain and regret than it's worth. And that's not just pain to yourself, but the people you are directly affecting.