These articles refute the myths that introverts are anti-social, shy, and wimpy. My resident introvert husband endorses both of them.
Carl King's review of The Introvert Advantage (How To Thrive in an Extrovert World), by Marti Laney, Psy.D. called 10 Myths about Introverts.
A funnier, snide 2003 Atlantic Article by Johnathon Rauch called Caring For Your Introvert.
There's a long 2010 Psychology Today article by Laurie Helgoe, Revenge of the Introverts, that has been endorsed by many clients, who linger in the waiting room after sessions to finish it.
Read at least one of these. It's useful to understand the temperaments of your clients, so that they can understand and accept who they are. There's a lot of overlap between introverted people and Elaine Aron's Highly Sensitive People, and those whom I call "skinny, nervous, people". I see every client through the lenses of temperament, attachment, trauma history, gender, class, and culture. We need to take all aspects of our clients into account, in order to fully see them, and treat them skillfully.