What does the letter represent?

The letter symbolizes misrepresentation. Mercier does not believe there is anything wrong with using his father’s letterhead even though his father is dead. He claims that it causes less confusion. It the case of Bertha’s will, however, the use of his father’s letterhead actually causes more confusion. Gamache is baffled not only that he is called to a home that appears abandoned, but that he was apparently called there by a dead man.