Sam's Journal, entitled "Letters to Katie", is a journal Sam kept to express her thoughts while her big sister and usual confidant Katie was away in Europe and unavailable to talk to. The entries are intended for Katie as their audience.
It is the last item to find in the game. Picking up the item will play the final journal entry, then roll the credits and end the game.
The entire game revolves around finding and picking up objects related to Sam's experiences while Katie was away, and having those objects trigger the reading of Sam's journal entries. But we don't find the actual journal until the end of the game. It's curious how Katie is able to know what the journal entries say well before the journal has been found.
In the attic, which can only be accessed after you find the Attic Key. It lies on a table with a desk lamp surrounded by three chairs or sofas covered in cloths.