Actually, a "book" is whatever can perform the function of tree bark (specifically referenced in the word is the
beech, fagus) - implying, writing on it.
This is very interesting, because since 'book' indicates the medium, not the content, and to refer to the latter through such word is a metonymy, it follows that 'eBook' indicates the tablet, the display, not the product of a writer. "Buying an eBook" literally means, buying the hardware.
Which raises the question of what should be, in the indoeuropean tradition, the name of the content, of the text. In this context heavily populated by terms that mean the technical, practical side ("holding a stick" for gramma, graphys etc.), the term 'text' is magic: clearly indicating the textile art (again technical), it was taken as the metaphor of "waving of ideas".
Today, the naming for electronic content can be found in the names picked for the file formats: a PDF is an instance of "portable document", an EPUB of an "electronic publication", and many formats are "text", "text documents".
Also: one cone of terms that sticks out of the technical side to mean the content, alongside 'text', as I could find after a quick search, is that of the "epistle": it means something that is "put in place" - which can be both the stele (well-in-place column) and the "established" message passed.