I've placed this question under the Speaker section, but it may be more appropriate here. The side walls of my listening room are lined with open back book shelves filled with books. When measuring speaker placement from the side "walls", do I measure from the walls behind the books or from the front of the shelves? I ask because the shelves on one side of the room are two inches wider than the other side.
I'd try Door Number 3: Measure it from the typical book spine. Also, don't sweat it too much. Given that you've effectively got irregular side walls, there's no way to do this to centimeter accuracy. And book shelves make great diffusers, so you're better off than folks with straight, hard walls.