As a disclaimer, I'm coming at this book as a youth sports professional which certainly colored my opinions. I thought it had some really good content, particularly the chapters on discipline and culture (though I was quite turned off by the line, "If you want obedience, have a child"). I appreciated Jenkins' emphasis on coaching as a partnership between staff and players, and her debunking the myth that authoritarian coaching is effective.
Unfortunately the bad content is Very Bad. I nearly stopped reading at the chapter on conditioning, which was full of inaccuracies and borderline bad advice. Jenkins is not a science writer, and while I think she was trying to simplify and possibly poeticize neuroscience the result was unclear at best and wrong at worst. There was a tone of victim-blaming and fat-shaming throughout the chapter as Jenkins implied that only people at their physical peak are capable of great things - and, presumably, people not at their physical peak are not capable of great things - and anyone with enough drive can be at their physical peak.
Wore than that was Jenkins' near hero-worship of Michael Phelps in this chapter. He is undoubtedly the best swimmer of all time, and he worked very hard to get there, but it is irresponsible to hold him up as the ideal when it comes to commitment and training. Yes, his punishing schedule allowed him to become the most decorated Olympian of all time, but at a significant cost. As a sportswriter, surely Jenkins is aware that Phelps has struggled with anxiety, depression, suicidal thoughts, and substance abuse problems. Inside the pool he may be the poster boy for hard work and commitment, but he has had a very difficult time figuring out who he is and how to function outside it. Phelps may have had mental health issues with our without swimming, but living his life in a pressure cooker surely didn't help.
In fact, only once did Jenkins acknowledge the possible downsides of this type of single-mindedness, when she mentions in passing that, "One cost of [Tom] Brady's intentionality was an eventual divorce." Otherwise, there is no discussion of the fact that many top performers suffer from mental health issues, stress-related illness, and failed personal relationships because their lives are utterly lacking in balance. Their is no recognition of the fact that for every Tom Brady or Michael Phelps, there are thousands of athletes who suffer repetitive use injuries, burnout, and isolation, not because they are too weak to do what it takes but because it's not a healthy or sustainable way to live.
I would hate for parents of youth athletes to read this book and come away with the impression that the path to success is a 365-days-a-year training schedule or practicing one's throwing motion in front of a brick wall so that anyone with flawed motion will suffer "raked knuckles and scabs." Children need rest, they need fun, they need downtime, they need friendship, and they need time to explore varied interests - in other words, they need to be humans and not just performance machines. In hockey we say, "All roads lead to beer league," and the same is true of every sport. Eventually every athlete will have to step away from the game, and when that happens they will need to have an identity beyond that of "athlete" in order to live a fulfilling life.