One point Cain makes about introverts is that they often express themselves more fluently in writing. For proof, simply look at all the Goodreads reviews of this book. One after another, reviewers who identify themselves as introverts go on and on! So, I'm not going to summarize the book here, only to share some of my strong reactions to it.
1. I appreciate the author's taking the time to tell extroverts, too, how they can take advantage of their strengths and learn strategies from introverts to help them cope with their weaknesses. If she had only boosted introverts' self-esteem, we might have thought she was overcompensating. As it is, we can take her claim that these are different temperaments with different advantages and drawbacks more seriously.
2. I also appreciate her study of how the Extrovert Ideal doesn't reign in Asian cultures, and in fact, may hold Asian Americans back in mixed company. I wish she would look at Jews, too. Jewish women are allowed to be much more extroverted than WASP women, and Jewish men are prized for being more introverted (in the sense of thoughtful, persistent, and scholarly) than the general idea of masculinity would permit.
3. Cain gives us a thoughtful analysis of when introverts should "to thine own self be true" and when they should "smile and the world smiles with you."
For the rest, please read the book!