Quiet: the Power of Introverts in a World that Can't Stop Talking, by Susan Cain. It's definitely worth a read if you are an introverted type of person. Or even if you're not... it might help explain to you why your spouse/friend/child prefers to stay in most of the time or tends to think more than they speak.
So... the gist of the book is based on the premise that the Western world tends to value extroversion to the point that we introverts grow up feeling like there's something wrong with us. We go to schools where talking (aka: class participation) is part of our grade and we work in offices that have no walls. We're made to feel like our desire to curl up and read a book or to leave a party early means that we're strange and inhibited and need to lighten up.
Susan set out to define introversion and takes a closer look at many psychological studies on the subject, as well as anecdotal evidence from her own life. It's rather fascinating all the things she discovers.
Like for example, most introverts tend to be "highly sensitive people" (here's a quiz for you to see if you are one). Not all introverts are HSPs and not all HSPs are introverts, but the correlation is pretty high. Basically the deal is that we are highly sensitive to stimulation (light, sound, noise, opinions), which is why we feel overwhelmed quickly in social or stressful situations. When we make a point to have "alone time", what we're really doing is finding environments and situations that are less stimulating. I never even realized that's what I was doing, but somehow being alone in the dark with my cat and a sappy romance movie always relaxes me. Now I know why! ;) This also explains a lot about me: my tendency to absorb the emotions of those around me (if you're in a bad mood in the same room as me, I will catch it as quickly as I would a cold), my tendency to like a room to be completely dark and quiet to sleep in, my tendency to take the lowest dose possible of medicine. Etc. Etc.
Extroverts, she says, have a much higher threshold for stimulation. It takes more for them to react to things. So they tend to have louder music, more parties, more diving into a situation without thinking about it. Whereas we introverts are careful because we don't want to short-circuit ourselves, so to speak, extroverts dive into life headfirst in order to feel it all more fully. (Again, not all extroverts are this way, but the correlation does seem pretty strong).
I can't really do this book justice in a blog review, so I highly recommend reading it for yourself. If you want the cliffs notes version though, here's Susan's TED Talk on the same subject: