I recently bought a book - Sketchbooks: The hidden art of Designers, Illustrators & Creatives -featuring a couple of sketchbooks from various artists, one of them being Johnny Hardstaff. His illustrations (and mostly his sketchbooks) are probably of the most interesting I've ever seen
"Once finished, once completed, my sketchbooks are worthless. In a way they are themselves filters. They filter away the whimsical, the faddish and the pointless. That way, the only useful sketchbook is the current one, which carries forth with it interesting trains of thought."