I must submit a third vote for Susan Cain's Quiet[1]. This book is a great revisitation (and arguably a more diplomatic treatment) of the topic of Anneli Rufus' 2003 book Party of One: The Loner's Manifesto[2]: introversion, lonerism, individualism, etc. While the latter - as its subtitle intimates - is far more geared toward introverts/loners, Cain's book is effectively an overview and comparison of both the inward as well as outgoing personality types.
For those who do not label themselves as loners or introverts I would recommend reading Quiet and perhaps skipping Party of One altogether. However, for the loner/introvert I would recommend Party of One as the prerequisite to Quiet.