Quentin Blake (Born 1932) is a prolific and iconic illustrator and writer of children's books. He was first published as an illustrator at 16 while still at school and has been working as one ever since. Recently he has started to do work on murals for hospitals. Here he describes his process and here are some videos, many of which feature him drawing and talking about how he does it. This is also an interesting article, particularly for references to his process while illustrating (including bits of acting out to get a sense of expressions and postures) and illustrating Michael Rosen's Sad Book.