The Notebook is a historical item which can be found by Karl in the prison camp during Chapter 3.
Location[]
From the start of the level, go left then up through the gap next to the medical building. Head all the way left in the next area past the latrines until you reach the fence and find this item.
Description[]
"Many soldiers had notebooks in which they wrote their diaries and memoirs. These notebooks provided historians with first hand account of the horrors of war experienced by soldiers."