Notebooks are items found in desks and bookcakes of many houses. You can use a Pen or a Pencil to write text on it. This is useful to write down important things to remember, like where to find loot you dropped. Empty Notebooks are similar to journals, however a notebook contains 10 pages to write on, compared to a journal which has 20. Doesn't reduce stress, boredom, or unhappiness.
It can be renamed and used to leave messages in Multiplayer servers.