I've kept a sketchbook for as long as I can remember. The practice intensified in undergrad and my sketchbooks became vital and dynamic extensions of my work. My studio practice changed in grad school and I started using softcover moleskins (they come in a 3 pack) as a sketchbook and notebook. I'd record critiques, observations, research, sketches and layouts. The sketchbook became less precious and more of a deposit of ideas and notes.
I've continued to use the moleskins post-grad school and they are currently filled with lots and lots of circles. The above pages are related to my horizon line series of woodblocks.