Quote from Lucius™ ManualEdit
"You can toggle on and off your notebook by pressing the N key. The notebook keeps track of chapter progress, character conversations and chore progress.
You can go into detail of the chapters by pressing the names on the left side of the notebook. They will contain thoughts of the progress and might be very useful for solving the puzzles."
The notebook is used for finding out latest logs, quest objectives and hints, as well as chore updates. You can also track the latest dialogues you heard.