Susan Cain is speaking up for those who don't necessarily want to raise our voices: "Today, " she writes, "we make room for a remarkably narrow range of personality styles. We're told that to be great is to be bold, to be happy is to be sociable. We see ourselves as a nation of extroverts---which means that we've lost sight of who we really are." (That is, because a third to a half of the US population is introverted.)
Quiet articulates well the cultural expressions of introversion and extroversion, and it emphasizes the importance of balance; mixing introverts with extroverts, each capitalizing on their own strengths to stem problems that might erupt from the overly narrow vision that results from groupthink. Moreover, it discusses ways to broach relationships, career, and more, taking into account that you might have to "code switch" to make headway on the projects you're most passionate about.
Most valuable, though, is the grace and compassion the message affords: for introverts and extroverts alike, Cain says, "Love is essential; gregariousness is optional. Cherish your nearest and dearest. ... Spend your free time the way you like, not the way you think you're supposed to. ... Whoever you are, bear in mind that appearance is not reality. Some people act like extroverts, but the effort costs them in energy, authenticity, and even physical health. Others seem aloof or self-contained, but their inner landscapes are rich and full of drama." Sensible, pragmatic, and reassuring counsel.