There, Bob described a term that was new to me, but one which instantly made sense. He pointed out that, when it comes to books, we are in an interregnum: a political term which describes the period of discontinuity between the reign of one monarch and the next (in this case, the dying reign of codex books since ±300 AD). Bob’s point was precisely this: we don’t know who the next ‘king’ is going to be when it comes to reading — and all of our present explorations in e-reading software and hardware are only baby-steps in this direction.
Interregna produce punctuated periods of intense creativity, problem-solving and social cohesion. Social scientists, theologians and philosophers have used the similar concept of liminality to describe the “ambiguity, openness, and indeterminacy” of our present age; we’re living in a “period of transition where normal limits to thought, self-understanding, and behavior are relaxed ‘ a situation which can lead to new perspectives.”). 
You can imagine that when cuneiform tablets gave way to animal skin and papyrus scrolls, and scrolls gave way to codices, there was upheaval and frustration with each new technology. Early adapters and stalwart, status quo types did their battles, prototypes were bandied about, and eventually the new became normative. But whole generations passed who lived in the interregnum — they didn’t know who the new king was, only, rather, that the old king was dead.
It’s an interesting time for me to be in publishing. I started off as a journalism major thinking I would make my living redesigning newspapers who were making the slow switch from manual to digital design and composition. When I made the switch to graphic design halfway through my university coursework, I kept that warm place in my heart for publishing, and many if not all of my best work in the last 20 years has centered around the publishing industry, whether it’s been periodical design, branding for publishers, transition strategies for publishers moving in to the digital space, tactical design for publishers wishing to broaden their footprint beyond their print pubs, audio packaging design, etc.
If you’re interested in this liminal space we publishing people find ourselves in, I encourage you to follow the work of Henry Jenkins , the Provost's Professor of Communication, Journalism, and Cinematic Arts at the University of Southern California. He’s a genius on new media literacy, transmedia storytelling, and the current changes in media consumption.
Here’s a great example of transmedia storytelling, but perhaps an even better picture of what we mean by interregnum. Here (below), the old is housing the new, giving credence to the new, and the new is tipping the hat back at the old (HT Bob Carlton):