Lights, Shadows and my self portrait a month later
The "last" chapter (there are actually two more, but they are more about applying creativity to other fields and some concluding remarks on finding the artist within you) of the "Drawing on the Right Side of the Brain" book by Betty Edwards focuses on perceiving lights and shadows, also known as seeing things in terms of "values".
This was another mind blowing chapter for me, I always regarded this part of drawing as something subconscious, and also simply I couldn't really understand how to use lighter or darker tones to give the idea of tridimensionality.