Letter to Ferg is a written note found in Dishonored.
A word of advice on your greedy little treasure hunting trips to the Flooded District. If you find yourself stranded and the only way out is through Rudshore Gate, this rewire tool will come in handy. If you can get through the doors, the circuitry panel for the wall of light is just on the other side.
Better yet, let me come with you. I'll handle the complicated things.
It can be found in the Flooded District in the locked room at the looters' hideout near the Rudshore Gate, next to the rewire tool.