I have a number of sketchbooks that I squirrel away in a cupboard once they are filled. They range from Moleskines in all sizes to larger sketchbooks, handmade ones, different papers, watercolour, cheap, expensive - they run the full gamut.
Ultimately they all serve the same purpose. They hone my drawing skills on a daily basis and it is a very rare day that I do not put something into one of my sketchbooks. I also have sketchbooks 'in progress' scattered throughout the house along with pencils or pens so that when the mood strikes the surface is always close to hand to capture the thoughts.
I don't always share the content of my sketchbooks online likely because its not finished work or even work in progress. It can be very loose lines almost like shorthand, readable only to me, sometimes with notes, sometimes not.
What do I sketch? People mostly as they're what interest me most. I have a little contest with myself to see how quickly I can capture gestures and lines before someone moves or before they figure out what I'm doing. Sometimes sketches or thumbnails make it into larger pieces, but often I simply draw for the sake of drawing, not with intention of completing it back in the studio.
There isn't often colour in my sketches either unless I decide that a sketchbook will be completed with only one colour pen or pencil. I've gone through my 'red' phase now and am considering the next sketchbook that I may get more creative with before I actually start it by colouring pages or adding media to the pages so each sketch in the future of that book will be a surprise to me.
I do like to look back over sketchbooks and they form a visual record of what I have seen and, like photographs, the images bring back memories of events and words.
I like to think I'm also creating a little bit of 21st century history for future generations who may look through these sketchbooks. They will be their windows to the past.