Tombow OnBook mechanical pencil is the perfect companion to a paper planner or Moleskine-type notebook. The pen is flat on one side to fit snugly against the spine of your book. The metal clip is solid and is very thin making it easy to slide along the spine of your notebook. It accepts any 0.5mm lead and is available in a half a dozen colors from JetPens. Under the cap is a tiny eraser which is not great but just enough if you’re in a pinch.
The OnBook is actually quite comfortable to hold for my small hands though I’ve never used it for long writing sessions. It looks sleek and it lives along the spine of my Cavallini & Co. leather-covered daily planner.