The dearest object an artist may have, is his sketching book. Besides sketching models as study for my paintings I do it for the pleasure in drawing. Working in a studio can be boring. Especially when the painting on the easel can be too difficult to succeed. For the moment you need to do something else than painting, to overcome the stress. Sometimes other obligations than art can be the problem. For that instances I always have my book and my pencil with me for that few minutes that suddenly occur. Wham.. Draw without thinking!
May be the happiest moments in an artist's life.
These sketches are what they are; Wonderful scratched gems, far from perfect, but precious to have and to realize on bad moments.
For my illustration work for magazines and books I prefer my pencil, because of the directness of expression and the tone of the pencil-line. You can see some samples at my illustrations page.

Realplayer 1.6 mb. 7 minutes Video Willem Mook's Schetching book