To me a book is a collection of printed words organized for reading and an ebook is simply one form of a book. I think even magazines could be considered books although they're a very different form of book.
Curious, I just looked up the etymology of "book" and it's thought that it's initial meaning was "tree", referring to birch trees in particular, since the early runes were printed on birch bark. Interestingly, the word "library" comes from a word meaning the inner bark of trees.
Anyway I doubt there's any kind of official dividing line saying what is and what isn't a book so we'll probably never agree on this. To make things easier I'll just declare that plain text is an ebook and that settles that.
Barry