This book appeared so many times in the movie Mank:
What's the significance of this book?
As written on it, it's a notebook. This hard-cover top-spiral model is still widely used, with or without brand/logo/texte on the cover.
As a screenwriter, you expect the person to have lots of ideas, lots of writing, and to put in on paper as soon as it comes to mind.
Just like a journalist, secretary, student... taking notes. If you focus on the drawings on the notebook in your picture, you'll see what looks like a meeting and a secretary, people taking notes.