http://www.amazon.ca/Quiet-Power-Introv ... 0307352153
This is a pretty good book. Talks about introverts and the true definition of what an introvert is... not the stereotypical, modern cultural definition where introverts are awkward, nerdy, glasses-wearing, comic-booking reading loners who stutter when they talk.
I would also say this book isn't just for introverts who want to learn about themselves. It's a good book if you're an extrovert who wants to learn how to interact with introverts, or vice versa.