A commonplace book is a resource or collection of idea’s, quotes, and observations. Commonplace books are believed to have been around since antiquity. Guides to writing a commonplace book, date back as early as the 17th century. The purpose is to keep hidden gems for use later. The commonplace book does not have to be a notebook but could be a series of index cards. The project is as individual as the writer. It could even be a board on Pinterest if you enjoy technology. I prefer to take notes and then add them later, you might have another method. Again this is down to the individual choice.