The Author, Lierre Keith, share her personal experience of being a vegan for over 20 years of her adult life and how it brought havoc upon her personal health and made her life so miserable. She does offer up some good points and information (which I enjoyed learning about!), but she's ready to "throw the baby out with the bathwater" in this book because the vegan lifestyle didn't work for her and she's bitter and has become an alarmist about it.
I myself, have been a vegetarian for over 25 years (of course, my vegan friends don't think that's good enough!) and was a top flight athlete, and continue to train and exercise everyday, along with a sensible vegetarian diet (a moderation of animal products, but no animal meats) and lifestyle, and that all works for me. Vegetarianism has worked tremendously well for me, but I am not extreme about it by any means.
The Vegetarian Myth, as the author writes about, is (in a nutshell) that too many people become vegetarians/vegans in hopes of saving the animals from exploitation, saving the environment, for their health reasons, and a variety of other things. Being vegetarian/vegan in itself isn't going to accomplish those things, but it can perhaps help in that direction.
The author obviously is upset that the vegan lifestyle didn't work for her and that she suffered and was miserable for 20 years and her health suffered. She also points out several times about how women are being exploited around the world, and while that's true, it seems to be a little out of context for this book.
My personal take is "all things in moderation", and if you're going to do something to an extreme, then you'd better really know what you're doing (it may just not be for you) and monitor yourself every step of the way. Good sensible diets, exercise and your own personal philosophy on life, plus "being the best you can be" is probably "just the ticket!".