As an intellectual object, a book is prototypically a composition of such great length that it takes a considerable investment of time to compose and a still considerable, though not so extensive, investment of time to read. This sense of book has a restricted and an unrestricted sense. In the restricted sense, a book is a self-sufficient section or part of a longer composition, a usage that reflects the fact that, in antiquity, long works had to be written on several scrolls, and each scroll had to be identified by the book it contained. So, for instance, each part of Aristotle’s Physics is called a book, as of course the Bible encompasses many different books. In the unrestricted sense, a book is the compositional whole of which such sections, whether called books or chapters or parts, are parts.
But the intellectual content in a physical book need not be a composition, or can be called a book. Books can consist only of drawings, engravings, or photographs, or such things as crossword puzzles or cut-out dolls. In a physical book the pages can be left blank or can feature an abstract set of lines as support for on-going entries, i.e., an account book, an appointment book, a log book, an autograph book, a notebook, a diary or day book, or a sketch book. Some physical books are made with pages thick and sturdy enough to support other physical objects, like a scrapbook or photograph album.
Books may be distributed in electronic form as e-books and other formats. In its “Revised Recommendation concerning the International Standardization of Statistics on the Production and Distribution of Books, Newspapers and Periodicals” of 1 November 1985, the United Nations Educational, Scientific and Cultural Organization (UNESCO) stated that it was “convinced that it is desirable that the national authorities responsible for collecting and reporting statistics relating to the production and distribution of printed publications should be guided by certain standards in the matter of definitions, classification and presentation,” and so “in order to improve the international comparability of statistics,” it defined a book as “a non-periodic publication of at least 49 pages exclusive of the cover pages, published in the country and made available to the public” (11(a)), so that books statistics could be collected on “(a) Government publications, i.e., publications issued by public administrations or their subsidiary bodies, except for those which are confidential or designed for internal distribution only; (b) School textbooks, books prescribed for pupils receiving education at the first and second level as defined in the revised Recommendation concerning the International Standardization of Educational Statistics adopted by the General Conference; (c) University theses; (d) Offprints, i.e., reprints of a part of a book or a periodical already published, provided that they have a title and a separate pagination and that they constitute a distinct work; (e) Publications which form part of a series, but which constitute separate bibliographical units; (f) Illustrated works: (i) Collections of prints, reproductions of works of art, drawings, etc., when such collections form complete, paginated volumes and when the illustrations are accompanied by an explanatory text, however short, referring to these works or to the artists themselves; (ii) Albums, illustrated books and pamphlets written in the form of continuous narratives, with pictures illustrating certain episodes; (iii) Albums and picture-books for children; (iv) Comic books” (9-10). A single sheet in a codex is a leaf, and each side of a leaf is a page.