John H. Vanderpoel's 1935 book The Human Figure: Life Drawing for Artists contains 430 pencil and charcoal drawings by the author and artist including a self portrait (see below). The book offers detailed descriptive writing on the the structural components of the eyes, face, hands, feet, etc with an emphasis on how the parts come together to make the whole. As Vanderpoel writes in his introduction: "Every stroke of the artist's brush[pencil] should prove his[her] understanding of the form of the subject-matter depicted" (page 13).
Lately when I'm not writing or working on a piece for my portfolio I pull Vanderpoel's book off the shelf and practice the structure and form drawings included in his book. Below are a couple pencil sketches, based on Vanderpoel's own drawings, that I did for fun including a rough reproduction of his self portrait.
In The Human Figure, Vanderpoel takes you first to what he describes as the "form and structural stability of the head" (in other words the skull; see page 17), and then to the eye. Here he writes detailed prose on the structure of the eye and I find his understanding of the forms within the form of the eye impressive.
The pencil sketches to the right and below are based on Vanderpoel's drawings from the book. I suspect that executing a slow careful pencil drawing would be a better approach to learning what Vanderpoel has to teach since a quick sketch relies more on mimicking Vanderpole's work rather than layering the characteristics of the structure to achieve a finished drawing that approximates Vanderpole's own illustration.
You'll notice a few notes next to the sketches such as "from memory" which means that I tried to build the sketch based on what I remember about not only Vanderpoel's description but also his drawing. I also make note at the bottom of the page of the hardness or softness of the graphite used for the sketch such as "2B test" or "6H test."