I found David Hockney's book Hidden Knowledge in the library last week and I've been pouring over it ever since. My understanding of art history tends to be quite patchy but I find it endlessly fascinating to learn about. Hockney's examination of the Old Master's and their (possible) use of optics is a good way in, and whether or not you agree with his conclusions it is an opportunity to examine the work by these artists more closely - as well as chronologically, side by side with their peers, which gives a little more context and room for comparison.
(Incidentally, I've also been captivated by the BBC series Bright Lights, Brilliant Minds: A Tale of Three Cities)
Some recent drawings from my sketchbook:
Drawn from Ingres' portrait of Mrs Charles Badham (1816)
Ingres focused most of his energy into drawing his subject's faces - they are drawn so tightly that you can barely see the individual pencil marks, the modelling of the face is often light and sensitive - while the clothes are drawn with more looseness of expression - harder strokes, stronger lines - still maintaining a strong focus on line and contour. This effect draws your eye to the face - it appears more in focus than the rest of the drawing - while the looseness keeps your eye engaged in the drawing as a whole.
drawn from Rembrandt's portrait of Baartjen Martens
(self-portrait with hair in towel)