I haven't heard the term commonplace book until today. It sounds like an art journal, a diary which includes drawings, maybe a scrapbook which includes things cut and pasted in from magazines and other sources. It is a curated collection of thoughts, ideas, and interests. The idea isn't so new but the phrase is new to me.

This description comes from a site which hosted commonplace journals. Its gone now.

A commonplace book is a collection of remarkable quotations, reflections, and oddities gathered from one's reading, thoughts, or experience--in short, a reading and writing journal.

A private commonplace book is also a great way to organize your research by author, source, and subject, and gives you the ability to display and search your notes by each category and by keyword.