Quiet - Susan Cain
I love pop science books, and when I saw this one was about introverts, I knew I had to check it out. I can't tell you how many times I was told as a child "people would like you better if you just talked more." Quiet explains the science of why introverts are the way they are, how to make the most of personal and business relationships if you are an introvert, or if you deal with one.
Susan Cain has an easy to read style, it reminds me of Mary Roach, though with a bit less humor. She uses personal stories and stories from her clients, and I appreciated the her references to scientific studies, which added validity to her observations. That said, you don't have to be a science buff to enjoy Quiet. I highly recommend it!