Source: Basics Product Design 01: Idea Searching, by David Bramston.
The sketchbook and notebook can work effectively in conjunction with each other, providing that sketches can capture the essence of what is being viewed, and the notebook can record it accurately and concisely. A few simple marks or key words can often record elements that a more sophisticated method of recording would not be able to do. The strength of a line or a metaphor can place an observation with surprising detail, which can be referenced at a later stage with considerable accuracy.
The designer needs to be a sponge for information and needs to be able to explore, analyse and categorise as frequently as possible.