Top Pixar animators Carlos Baena and Victor Navone have written invaluable information about how the eyebrows are useful in conveying what a character is thinking and feeling. They discuss different brow shapes and uses, and give several examples of visual reference from drawings and acting clips. These are great resources for animating faces... check them out and book mark it for the future.
Victor Navone: The Brows Have It
Carlos Baena: Eyebrows
John K. Stuff- Hierarchy- 3 Eye Area- eyebrows, irises, wrinkles