Drawing is Thinking is a short but lovely book Glaser wrote all about the practice of drawing in his normal life. He's drawing constantly, hes drawing the same subjects over and over and over. Obsessively. This is just part of his practice, part of this life. Drawing to get ideas out and crystalize them for himself. In the way that I draw, or take notes or Journal, its just an essential act of making sense of the world.
I had a drawing instructor in school who told us that Drawing was really just the Art of seeing, of truly looking at something and seeing what is really there. Which is a fight in your mind of what you think the thing looks like, and what it actually looks like. We have to train the eye to simply push away our abstracted ideas of what is there, in favor of what is actually there.