A List Apart has a great essay by Craig Mod about what makes a book a book:
In Hochui and Kinross’ Designing Books, they discuss the uniqueness of book symmetry: "The axis of symmetry of the spine is always there; one can certainly work over it, but not deny it. In this respect book typography is essentially different from the typography of single sheets, as in business printing, posters, and so on."
The spine gives book reading a kinetic motion not found in unbound sheets of paper. Forward and backward movement within a book happens because of the spine. And so designers erect scaffolding—text blocks and running heads and other literary accoutrements—around this keystone axis. It is the natural balance point of a spread. The implicitness of this means publishers have largely achieved functional book design right from the beginning: the forty-two line columns of thick type in the Gutenberg bible, even today, are quite a marvel of typographic balance.
And from there he launches into the idea of e-books and how the fact that they've been freed from the spine means that designers need to find a new kinesis. It's interesting lunchtime reading.