I was driving home last night and heard Jeffrey Hamburger, Kuno Francke Professor of German Art and Culture at Harvard University, lecturing on WBUR's World of Ideas broadcast. He talked about how, often, what is left out of discussions about the still-to-be-improved-upon design of the traditional book is the idea of the FACING page.
The basic visual unit that structures our experience of the medieval book is the opening. From the origins of codex as a medium in late antiquity, and in contrast to the scrolls used in the ancient world, the confrontation of the verso and recto provided the visual field within which scribes and illuminators operated. Openings also made possible the visible elaboration of the word with figurated initials, frames and full-page miniatures.
You can hear the lecture here.