Velvet Assassin

Letters are scattered all over, you must collect them on a single playthough, you can't reload a previous mission as this isn't a stackable achievement it must be done in a single playthough.

Maps with all the locations are under the "Eagle Eye" achievement listed above. Each letter is marked with a quill on the map.

Letter 1:
Location: The Lost, Fuel Storage - Can be found on a desk.

Letter 2:
Location: Death Of The Butcher, Crypt - On a small desk on your exit to the level on your right side.

Letter 3:
Location: Leave A Light On, Docks - In the middle of a room with an office desk.

Letter 4:
Location: The Third Man, Sewers - On top of a crate located next to a ladder.

Letter 5:
Location: The Third Man, Ghetto - In a small room which is opened with a key, the letter is on a small table in the corner.

Letter 6:
Location: The Third Man, Prison - On a bed in a cell located on the first floor.

Letter 7:
Location: Le Boeuf - Found in the basement of the Boathouse.

Letter 8:
Location: Fade To Grey, Hospital - Found on a desk which has a corpse in the chair and a wardrobe behind the desk.

These are just rough descriptions, use the maps provided under "Eagle Eye" to find each location.