Interesting question. I think we are in for the unexpected. Book reading is not a homogeneous phenomenon, in which one technology will oust another.
Instead, some of functions which books had performed will be taken over by the web (e.g. Wikipedia), other functions by apps (e.g. dictionaries). Novels and popular nonfiction are probably the most likely to come out as e-books. Many technical and scientific works will probably continue to appear as PDFs.
Books which could be considered subversive might appear in paper form so as to avoid government surveillance. Textbooks seem to have certain advantages in paper - easier to flip back and forth, for example.
New media usually don't totally replace old media. Instead, each specializes in what it does best. Movies and radio lived on after television - and so it will be with p-books and e-books.
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