I'd say that, to be a book, whether material or virtual, there are a few basic qualifications -- I can think of six off the top of my head:
• A book must contain readable text.
• It must be portable -- the ability to take a book anywhere is one of its key strengths.
• The text must be persistent -- that is, it should still be there if you go away and come back again later.
• You should be able to do what you want with it: store it, loan it, give it away, bequeath it, and (yes) destroy it if you have a mind to.
• It should be able to be annotated, written in, drawn in, dog-eared or place-marked. Call it "interactivity" if you like.
• It shouldn't vanish unexpectedly. And, if undisturbed, it should last for years.
So is a typical e-book a book by these measures? In most cases, no. It meets the first two criteria, yes -- but is it persistent? Some e-books leant by libraries expire after a certain date and can no longer be read; some e-books can only be read in certain places (as with Barnes & Noble's 'share-in-store' café feature) -- that's not real persistence. The fourth qualification, though, is the biggest stumbling block: almost no commercial e-book format allows lending or giving of any kind. If, in a lifetime, you amass a library of physical books on which you spend tens of thousands of dollars, you can give it to a friend, leave it to your kids, or donate it to a library. If you did the same with e-books, you'd have nothing -- your death would be the death of every e-book you'd bought.
Annotation? Some platforms allow this, and there's even one model in which one can see other peoples' annotations -- wow, just like a book! There are "signed" e-books never touched by an author's hand. But if the lifespan of an e-book is uncertain, the duration of these user-added annotations is even more questionable.
And disappearing? Amazon.com famously deleted copies of Orwell's Animal Farm from its users' Kindles, kindly crediting them 99 cents, after the company was informed by the Orwell estate that the book was still in copyright -- talk about Orwellian. And there's nothing to say Amazon or some other vendor couldn't do it again. What's more, if you decided not to be an Amazon customer, or not to replace a broken Kindle, or if Kindle were to be replaced by some hardware or software that wasn't backwards-compatible with older e-book formats, then your books would have vanished for you.
Lastly, what would one make of some archaeologist of the far future, coming upon a buried e-library? If Amazon didn't exist in the future, there'd be no way to recover these battered e-readers and tablets -- their data was mostly stored on cloud that once floated in the sky of a lost civilization. And like clouds, there'd be no getting them back.
So I suggest a label, or some sort of certification: Only e-books and readers that met the six criteria above would be certified as genuine "books" -- everything else would have to use some other word: text-sacks, wordblobs, readoids, or libri-fizzles. Anything but "books."
(illustration from wikimedia commons)