Leah Price’s answer is more complicated than you’d expected:
“[B]ook” has meant different things to different cultures. It sometimes refers to a sequence of words that’s long enough to form a whole, though short enough that it’s not impossible for one person to read the whole thing—and even then, most books are treated more like a buffet in which readers graze for quotable quotes than like a meal whose different courses need to be eaten in sequence and it’s an insult to the chef if you don’t finish everything on your plate.
At other times and places, a book hasn’t referred to a sequence of words but rather to a material object—a roll of a manageable size, for example, or a codex that’s thick enough for its title to be spelled down its spine. This is one reason that UNESCO defined “book” in 1964 as “a non-periodical printed publication of at least 49 pages, …made available to the public”; but the redundancy of “publication made available to the public” points to what remains slippery in that definition. Where do you draw the line between publication and internal circulation, for example? And “of” begs questions as well. The early modern Sammelband is a book made of different works bound together by what we would today call the end user—it’s the reader, not the publisher, who assembled those parts into a whole that we’d recognize as a book. So “book” is a term that has fluctuated and been fought over long before e-books began to face off with p-books.