Employer: Basso [Stonemarket - Basso's Office]
Availability: You receive the opportunity to start this mission, after you have completed the game's second chapter. You need to meet up with z Basso w in the Crippled Burrick tavern.
Your destination has been shown in the above screenshot - it is located to the North-West of the tavern and to the North of Ector's workshop, in [Stonemarket - Baron's Way North]. Note - Before you get there, it is a good idea to get a Razor, because this will let you acquire all of the items available to steal in this mission.
Just as a reminder, you can get to the Baron's Way North by finding the narrow passage, to the North of Ector's place, and by squeezing through the crates. Find the Leatherwood Furniture building, which belonged to the deceased Christina and pick the lock on the door, thanks to which you will be able to start the exploration of [Stonemarket - Leatherwood Furniture].
At the building's upper floor, you will find a Candlestick - 15G. Climb down now, to the cellar and search out a small venting shaft hidden behind barrels. After you have reached the neighboring room, collect the Pile of Coins - 1+5G and locate the button attached to one of the barrels, which opens up a secret chamber. Jump down to the secret chamber, to find a Poppy and a painting (The Court of Montonessi - The Insatiable Appetite), which you can obtain only by using the Razor.
Return now to the cellar's first room and start climbing in the spot shown in the above screenshot. Under the stairs, you will find a chest of drawers, where there is the last will (Stonemarket - Lady Christina's Will - 50G) that you are looking for. At this point, this mission ends.