Complex sentence: At any given moment the entirety of a book exists in itself but at any given moment we can only read one page at a time, the other pages, while existing simultaneously with the one we are reading, remaining outside our consciousness.
Split into simpler sentences: At any given moment the entirety of a book exists in itself but at any given moment we can only read one page at a time. The other pages, while existing simultaneously with the one we are reading, remain outside our consciousness.