Ekman is a leading authority on the study of the facial expressions and their relation to emotion, and this book is a methodical and thorough introduction to the field, with special focus on recognizing what he calls the six basic universally expressed emotions: happiness, fear, anger, surprise, disgust/contempt, and sadness.
Ekman provides clear, well-detailed instructions for recognizing each of these emotions, and the book includes plenty of photos to illustrate the different facial contortions of each emotion. The only complaint I have is that I had to do a lot of page flipping back and forth to read Ekman's remarks and subsequently refer to the relevant picture. Perhaps a future edition could be better formatted to make it easier for the reader to view the expression alongside Ekman's comments.
In addition to writing about specific emotions, Ekman also covers some fascinating related topics such as recognizing facial deceit and discovering the patterns of one's own facial expressions, i.e., what you're telling the world with your own face.
Ekman is an academic and his writing shows it; he's precise, methodical, thorough, and careful in the extent of his claims. Readers who are new to the subject of reading facial expressions but are seriously committed to learning about it will find this an invaluable book.
(Ekman's later work, "Emotions Revealed," is also a great read and contains much of the same information as "Unmasking the Face," although I found the former to be lighter on technical information and practice faces, and more focused on the larger reflections Ekman has made looking back on his work over the last few decades.)