While gathering reference for my projects I found FaceTurn a Web site which lets you view a variety of expressions using a rotatable and zoomable 3D model.
I don’t know if it’s intended for 2D or 3D artists, but I like it for several reasons. It lets you select from a long list of facial expressions. Each one has a 3D model that you can zoom and rotate any way you wish. It also gives info and characteristics of each expression. Other features found with expressions include “fun facts” and 3/4, straight on and profile views of the expression. It’s definitely a nice alternative to somewhat cumbersome books.