mmonika You could do 2 different depths..
All of the lower cabinets inline with the window seat be deeper, which would be great for storage of those bigger items but tier the upper shelves so their depth isn't so vast. This will also, visually, not seem as heavy.

To address your question specifically, consider what you will store and make your depths as needed ( with false backs or not). Sometimes there are pieces which just don't fit in the shallowness of bookcases and that would be annoying.
If you want to be flexible with depth, use some boxes (wood, card, papered, painted..etc) in behind of shelves you don't want to lose books in but still can opt to store your board games :-)

dmsutton I don't know about the USA versions of these houses but in Australia most of them are cavity brick walls which are at least 10 inches (250mm) thick and this would not require any false backing. The depth of the reveal as depicted would be correct as such a window is mounted on the exterior brickwork. This mean with the depth shown the shelve would approx 8-10 inches deep.