I like Bob Stein's concept of "books" as "user-driven media"-- it validates something I understood intuitively when, as a child, I owned a Game Boy and attempted to convince my 3rd grade teacher to let me play Final Fantasy Legend III during Quiet Reading Time. She didn't understand that I was engaged in a complex narrative about redemption, sacrifice, time-travel, a flooded-world apocalypse, and yes, one that also involves fighting cyborgs and monsters in simulated battles, but that's a crucial part of the protagonists' heroic struggle.
What books and games have in common is that they are user-driven media, and in my opinion the "Future of the Book" is already intersecting with the interactive media experiences pioneered by video games-- or maybe it always has been so (Stein did work for Atari's Digital Books Division, after all) and only now is there the technological foundation for widespread acceptance of the artform. The tools for creating interactive experiences are rapidly becoming available and accessible to everyone-- Stein's prediction that "Authors will have a different set of skills than we associate with them today" is already reality, and if anyone needs proof I submit for your approval Heavy Rain. The best way I can think to describe it is as a 3D interactive murder-mystery; it is a narrative which requires player input, and the consequences of actions over time have a significant impact upon how the story unfolds, especially the ending (reminds me of the IDEO Future-Book-concept "Alice"). Plot and character development are as central to the experience of Heavy Rain as is the gameplay-- in fact they go hand in hand, with aspects of gameplay often performing unique narrative functions (for example, holding a button will allow the player to hear what the protagonist is thinking at any particular moment).
The creation of "books" like Heavy Rain is, nevertheless, a process involving extensive production teams with various skillsets. It may always be the case that such interactive experiences will never be the product of one individual author, which is a serious departure from how narratives have been traditionally produced. Perhaps if the "author" comes to occupy a position more like "director", or, as Stein terms it, a "knowledgeable moderator", such narratives will retain a degree of individual authenticity.
The developers of new technologies nowadays seem to recognize the importance of making tools accessible to users, something which Stein clearly values greatly. Just recently I discovered a Kickstarter page for a product known as the "Rift", which is basically a virtual reality headset that doesn't suck. The company behind it has achieved beyond its original funding goal for producing "development kits" to send to game developers so that they can become familiar with the device-- their emphasis on making the Rift accessible to developers is the biggest reason I think the product will be successful. Although creating fully immerse, virtual-reality experiences is still beyond the reach of your average author, it is only a matter of time before the tools of their production become commonly available (as is now the case with video/photo/audio editing). I'm looking forward to the results~