Applied Illustration: Portrait Reference

SIMON, Mark, 2005, Facial Expressions: A Visual Reference for Artists, New York, Watson-Guptill Publications

I have referred to this particular book on more than one occasion. Peculiarly, the book includes examples of artist's work based on the reference, which are mostly terrible.
The reference imagery uses a mixture of models, ranged in race and age. This is quite useful considering the range of pilots I wish to portray.