This book appeared so many times in the movie Mank:
What's the significance of this book?
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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.
On your screenshot, notebooks are even piled up, making them look like a "library of thoughts". They were just the forebears of modern electronic pocket recorders.