If you’re interested in learning more about the theory and practice of sketching outdoors, I can recommend reading Field Sketching and the Experience of Landscape by artist and landscape architect Janet Swailes.
It’s a thoughtful and fascinating book that explains how the action of field sketching affects and enhances our perception of the environment; and how we can use sketching as a tool to communicate our experience of landscape.
Field Sketching and the Experience of Landscape is primarily aimed at landscape architects and is not an easy ‘How to Sketch’ beginners’ guide. Nevertheless, it contains a wealth of practical tips and inspiration for keen Boggy Doodlers.
This is also the book that helped me understand why walking plays an integral role in the sketching process (walking enhances our perception of our surroundings) and is why I always try to incorporate a bit of a walk into every Boggy Doodles event.