This letter, or rather these letters, as there are two with this title, is a quest item obtained in Chapter III. It is a pass which allows Geralt to roam about the Trade Quarter during the period of martial law without being unduly harassed.
Pass obtained from Triss Edit
The first pass is given to Geralt by Triss Merigold at the very beginning of Chapter III when he wakes up at her house in the Trade Quarter. She tells him that she falsely registered him as a resident at her address.
There is no visible signature on the letter.
Pass obtained from the Order militia sergeant EditThe second pass is optional and can be obtained by talking to the Order militia sergeant stationed close to the town hall at night. He is easily distinguishable from the other guards in his red tunic.
There is no visible signature on this letter either.