If you want to go deeper into this, there is the Manual that describes the action units and how to measure them, but you'll also want to read about how specific emotions manifest in humans. For that, some other Ekman's books are useful, in no particular order:
- Unmasking the Social Engineer: The Human Element of Security by Christopher Hadnagy, Paul Kelly F. & Paul Ekman (2014)
- Unmasking the Face: A Guide to Recognizing Emotions From Facial Expressions by Paul Ekman & Wallace V. Friesen (2003)
- Emotions Revealed: Recognizing Faces and Feelings to Improve Communication and Emotional Life by Paul Ekman (2003)
- Telling Lies: Clues to Deceit in the Marketplace, Politics, and Marriage by Paul Ekman (1992)
- Emotion in the Human Face: Guidelines for Research and an Integration of Findings by Paul Ekman, Wallace V. Friesen, Phoebe Ellsworth, Arnold P. Goldstein & Leonard Krasner (1972)
- What the Face Reveals: Basic and Applied Studies of Spontaneous Expression Using the Facial Action Coding System (FACS) (2nd Edition) by Paul Ekman, Erika L. Rosenberg (2005)
