what is sketchbook?

A sketchbook is a "a book or pad with blank pages for sketching," and is frequently used by artists for drawing or painting as a part of their creative process. The exhibition of sketchbooks at the Fogg Art Museum at Harvard University in 2006 suggested that there were two broad categories for classifying sketches

Observation: this focuses on the documentation of the external world and includes many such travel and nature studies and
sketches recording an artist's travels.

Invention: this follows the artists' digressions and internal journeys as they develop compositional ideas