There is clearly a lot of interest in modelling faces in great detail among ZBrush users, and I wondered if y'all were aware of a book by psychology professor Paul Ekman called Emotions Revealed. The ISBN number is 0-8050-7275-6.
No, it's NOT a self-help book -- it's about the physiology of the face and how we use facial muscles to express our feelings, consciously and unconsciously. Ekman has done work on the difference between hard-wired facial reactions and those we acquire culturally, whether to express emotions or to mask them. He has gone so far as to use an electrical stimulus needle on his own face in order to map all the facial muscles and how each one affects our expressions!
So this book contains extremely precise description-maps for constructing a face into an emotion as well as a lot of interesting (to me anyway) background about emtions in different cultures.
I mention this book because I see a number of face/head models in this forum. They are extraordinarily precise in their topology, but sometimes the expressions are incomplete, like only moving the mouth to make a smile. This book might be the reference you need to make your faces even more true to life.