Here’s one definition of a commonplace book:
I used that definition mostly because I didn’t want to publicly resort to Wikipedia.
The term has come up twice in my recent reading. The first was in the most recently read Dance to the Music of Time novel, Temporary Kings. A now (prematurely) dead novelist name Trapnel X about whom a fellow named Gwinnett is writing and autobiography, kept a commonplace book. Then, I saw this reference to commonplace books in an interview with the poet-cum-artist Donato Mancini.
I had mostly just taken by context what a commonplace book actually was until the second encounter with the term, after which I figured I had best find out for certain. While learning did not really change my understanding of what I had read, I did give me a name to attach to what I had been doing (and I’ve read enough Ursula K. LeGuin to appreciate the power of names and naming). My vast array of notebooks, scattered across both my home and (likely) two or three continents are, in my use of them, commonplace books.