If desired, the curtains on the built-in bookcases can be pulled shut to eliminate visual clutter. Also, the bookcase curtains have exactly the same treatment as the window curtains, providing continuity throughout the space.
Both continuity and elimination of visual clutter can help small spaces seem less crowded. And it's a great way to add storage without necessarily putting it on display. This same idea could translate from book shelves to a pantry shelf housing all of your dish- and cookware, to clothing storage in a closetless bedroom, or to A/V storage in the living room.
See the rest of the townhouse where we spotted this idea at Barry Dixon Inc..