Drawing facial expression is very important for an artist. It helps you to show the range of human emotion in your art. When I was looking for books which would help me to get the facial expression drawings right, I came across two highly recommended books to study from. One was"Facial Expressions: A Visual Reference for Artists " by Mark Simon and the other one was " The Artist's Complete Guide to Facial Expression" by Gary Faigin. But before that, I would recommend reading human anatomy books for learning to draw human head first. It gets easier to draw various expressions once you are aware of the underlying bone structure. Some of the best books to read for drawing human head would be Andrew Loomis' "Drawing the head and hands" and "Figure drawing for all its worth". Andrew Loomis books are freely available online. You can download all of them here. Apart from Loomis, books written by Burne Hogarth, George Bridgman, Jack Hamm, Stephen Roger Peck, Eliot Goldfinger etc are also highly recommended ones. It's easy to get overwhelmed by this huge list of books. It all depends on whose approach you find easier to follow. Every book, whether it falls on the recommended list or not, teaches you something or the other. So never underestimate any book. To become a better artist one must have a "sponge-like" attitude. Soak in all that bits and pieces of information you come across in your art journey.