If you are interested in learning more about the Sight-Size approach to drawing that was used to train artists in the 19th century, I highly recommend Darren R. Rousar's highly informative book titled: Cast Drawing: Using the Sight-Size Approach. You can order Darren's book directly from Amazon.com. There is a companion DVD also, Sight-Size and the art of seeing. These two items will get you started in the art of classical drawing methods.
Book Title: Cast Drawing, Using the Sight-Size Approach
Author: Darren R. Rousar
Velatura Press, Minneapolis, Mn 2007