I have many sketchbooks for different... creative ideas? 

Big ones, small ones, square, rectangle, portrait and landscape, the differences in them allows my brain to think in different ways somehow to create colourful and detailed full imagery. Sometimes I just use my black pen other times it's marker pens, coloured pencils, inks or paint. These pages are purely for fun and creative release, when creativity is your job a sketchbook is like a playground!