I think this book is a must-read for... well, for everyone. 
 Introverts should read QUIET because it will help them understand themselves better, as well as the way people around them respond to them. For me, at least, it explained a lot. QUIET is one of the few books that helped convince me that I am not in any way "wrong" or "strange" and that it's okay for me to be myself. (I would especially recommend this book to introverted adolescents. High school is hell for most introverts and the wise words of Susan Cain might just help some of them through it.) 
 Extroverts should read this book, too, because it will help them understand introverts better. That is something which I think our society could use - more understanding of introverts. As Cain states, most cultures and societies across the globe are currently aimed at extroverted people, with introverts - the ones preferring solitude and quiet over socialising and lots of stimulation - often being considered outsiders - or at least they are not seen as the norm... and that while a third to half of all people are introverts. 
 Susan Cain provides the reader with tips - tips for introverts, tips for extroverts to deal with and understand introverts - that I believe can literally change your life. She gives real-life examples of herself and other people, she supports her statements with scientific research, and she even touches the subject of high sensitivity (which is, by the way, also something I believe everyone should know about, but that aside). It was a little slow reading at times, but the insights in this book are important. I, at least, have learned a lot from them.