HI!
I like your drawings. Shading the 20 Minute-works - good! Do you have a good book on Anatomy? I can recommend you "figure drawing - design and invention" by Michael Hampton. There are basic guidelines for drawing anything of the human anatomy. There I found the planes of the face/ head, which help immensely building depth to the human face. I think the book is for 2,99€ amazon kindle...
Michael Hampton Figure drawing
