/ic/ - Artwork/Critique

Repostan
Where can I learn how to shade the face better? I know the location of the bones and muscles in terms of line, but I clearly still have only a simplistic idea of where to place shadows. Where can I read some really in depth material on how to shade the different planes and muscle areas? Clearly some shaped are rounded and others are flat or sharp, but Loomis doesn't go in depth on that

I am having trouble giving the planes of my face proper shading.
I don't know how sharp my core shadows are, or where to put them.
I know where the bones and muscles are located, and I can roughly understand what the head looks like in perspective.
I want to know where I can read up and study the details of the face, and how light hits the different muscle groups, which I already know the location of. I also wonder if there are other details in the face I am missing out on.
I don't want to just study portraits because I want to be able to draw and shade faces from imagination. I want to know in depth about the anatomy of the face and how it is lit.