Facial Expressions

Understanding Comics and Making Comics by Scott McCloud are both about comics but are actually very relevant to the animation artist as well. They are both in comic book format (way more fun than the typical textbook) and well worth the read.

From Scott McCloud's "Making Comics"

Based on McCloud's work on expressions, The Grimace Project is on online expression generator. You can see how any 1 or 2 emotions impact the facial expressions. I don't think it is always 100% accurate, but this could be a great resource when you are trying to create different facial expressions.