A book is a set of written, printed, illustrated, or blank sheets, made of ink on paper, parchment or other materials, usually fastened together on one side. A single sheet within a book is a leaf, and each side of a leaf is ... wait for it ... a page. While these are all parts that you won't find in the actual sense in an eBook, this is just the latest version of "the book" throughout history.
When writing systems were created in ancient civilizations, nearly everything that could be written upon - stone, clay, tree bark, metal sheets - was used for writing. From clay tablets to wax tablets, from scrolls (say hello to the Egyptian papyrus) to the Codex which was the first format that is similar to out modern books. The Middle Ages welcomed parchment and calf skin in manuscripts, and today we have already moved from good old paper to digital books.
Actually, when you think about it, it might have taken a long time, but we managed to move from one tablet ...
Copyright by Wikimedia
... to the next with ease.
Copyright by TeleRead
Of course a library filled with clay tablets would be a wee bit larger than the library on your eReader. On the other hands, those clay tablets will still be there in a 1.000 years while the eReader will be ancient history. That's progress for you!