It looks like you have a pretty good grasp of the head! You have some good stuff going on there! You may find studying the connection between the neck, ear and jaw helpful, I feel like give a good anchor for the bottom half of the head, but what do I know XD I think that generally looking at the skull and planes of the face really helps, but also I personally like to copy really good sketches of faces that help me figure out what lies to pick. It seems like you already have done these things though :) oh! "Drawing expressive Portraits" by Paul Leveille is a good book
