This is an interesting one, sure.
Hot/cool is an interesting approach, but I think Clay has the hot/cool reversed: cool media are ones that require active participation, and usually hit more than one sense at a time. Hot media are more engrossing, and less participatory. McLuhan saw radio as a hot medium, because it was high fidelity but hit just one sense—hearing. It was so hot, in fact, that Hitler was able to stoke his mob this way. Television, on the other hand, was a cool medium. It was grainy back then, and required more participation in order for the viewer to resolve the image. Kennedy, the "cool" candidate, did better than Nixon, the "hot" candidate. (Likewise, Obama, a cool candidate, did better on TV than Hilary, a hot one.)
The book is engrossing and uni-sensory, so it counts as hot in its current form. No participation, just engagement. We are swallowed up by the book. As the book becomes more digital, we tend to click around more, we have hyperlinks, we even have the ability to discuss the book with friends and peers as we read. These all contribute to making the book a more participatory and cooler experience. We can have more distance, we are alienated from the passion of the text to some extent, and we are connected to other readers. It's a bit like watching Mystery Science Theater 3000, where you have buddies to comment on the action of the B-movie within the screen. A book in a Kindle is a bit like a screen within a screen—a Brechtian alienation effect that makes us all the more aware that we are reading a book. This puts us in a more objective, cooler posture—more conscious that we are interacting with a medium.
But I think if McLuhan were to be required to say something about ebooks, he'd probably just apply the tetrad: What does the medium amplify? What does it obsolesce? What does it retrieve? What does it flip into when it is pushed to the extreme?
It's quite early, so it's tricky to know how to answer these.
• My sense is that the ebook amplifies sharing and shared experience. Every book is a link, and a book club to be engaged in together.
• What does it obsolesce? Most simply, the printed book. Among other things, it obsolesces industrial production, and the centralized top-down control that goes with it. Everyone is a printer and publisher now.
• What does it retrieve? I'd have to follow Clay's lead there: I think it retrieves some elements of peer-to-peer oral culture. To "read" the book is now to engage with others about the book. It's more a conversation with fellow readers than a solo relationship with the author.
• The hardest one is the last one: what does it "flip" into when pushed to the extreme? I might just argue the end of authors. Since a writer no longer needs a publisher, everyone can be a writer. Furthermore, the digital document is read/write compared with a book, which is bound and done. Just as the net challenges central authorities, it challenges personal, solo authorship and the career one could have by claiming authorship.
• I think that's what McLuhan would have ventured.
ps. The other thing McLuhan would have noticed is that the ebook creates order out of chaos. The scattered library immediately becomes a list that can be ordered and re-ordered along almost any parameter. "Mechanical Bride" was originally going to be called "Guide to Chaos," remember. This chaos of the industrial press is obsolesced by the order of the digital book—which isn't really a book at all, but a new kind of library.
I concur with Nick's suggestion that we don't yet know the form of e-books, because they are still mainly containers for the textual residue once the pages of books have evaporated, and that reading will become (has become) a more confusing experience. (There's an interesting musing on Nick's work here:
What's next though, in McLuhanite terms, is up for grabs. I think the most apposite McLuhan quote is "Even I don't agree will all the things I say." The man wrote long enough and widely enough that there's often a potential conflict in his thought.
In this case, reading is moving from a 'cold' to a 'hot' medium, analogous to the switch from radio to TV, and is becoming more tribal—I know of several organizations working on 'socializing' reading, and every e-book reader now includes ways of sharing user annotations widely—both things McLuhan predicted.
However, I think e-books might also have surprised McLuhan—an under-appreciated aspect of his thought was his Catholicism, and he regarded the Protestant view of the primacy of the book as a historical anomaly, and assumed that electronic media would continue its march back to the land of imagery. I think 4chan would have puzzled him less, as a cultural product, than Amazon Singles.
McLuhan pointed out the initial content of a new medium is the old medium it replaces, and we seem to be in that phase with ebooks—the content of today's ebooks is print books. What that also means is that we don't yet know what an ebook really is, because it has yet to take its true shape. But there is an important hint: we can see in the web itself what a computerized, networked, screen medium looks like, and that's likely to be a closer model for the ultimate form of an ebook than an old printed book is. So if a printed book served as, to borrow Robert Frost's phrase, "a momentary stay against confusion," an ebook will likely serve as a further indulgence in confusion. (And I mean "confusion" in all its meanings: a mingling together, a jumble, bafflement.)