Quiet has been the one life-changing book for me. I read it at a time when I thought of myself as unsocial (because I didn't like parties) and was uncomfortable expressing myself even among a large group of friends. After reading that book and realizing how widespread introversion is, I became more and more comfortable in my own skin. Ironically I also started enjoying more at parties because I wasn't judging myself constantly, which helped me become more of a pseudo-extrovert when I needed to be.