Complex sentence: Swann wrote that he was aware of most of the objects on shelf above his head, but he did not know it held four numbers on a side that would "not" have been visible if a reflecting surface had been angled near the end.
Split into simpler sentences: Swann wrote that he was aware of most of the objects on shelf above his head, but he did not know it held four numbers on a side. A side that would "not" have been visible if a reflecting surface had been angled near the end.