Marti Olsen Laney - Amazon page.
Lee Ann Lambert - Amazon page.
Leil Lowndes -
Cool Communicating -
Not specifically about introversion, but may be helpful for some introverts. (Her remarks about PEA (phenylethylamine), and romantic love lasting only one and a half to three years, really got my attention... whether or not this explanation is correct, I'd already noticed that three year limit in my own relationships... time and again, I've seen changes at about the three-year mark!)
Leil Lowndes - Amazon page.
Quiet: The Power of Introverts - Susan Cain's site.