Book structures, by their inherent physical characteristics, lend themselves to communicating certain specific types of narratives and should therefore be organically linked to the book's content.  Accordion fold books tend to represent linear flow, often found in timelines or books with smooth forward moving sequences.  Book objects are often highly sculptural or material representations of the content of the book, either overtly or by inference.  Sometimes book objects can become a little gimmicky, but they usually provide drama and strong impact.