A book is both a usually portable physical object and the body of immaterial representations or intellectual object whose material signs—written or drawn lines or other two-dimensional media—the physical object contains or houses.
As a physical object, a book is a stack of usually rectangular pages (made of papyrus, parchment, vellum, or paper) oriented with one side (either left or right, depending on the direction in which one reads a script) tied, sewn, or otherwise fixed together and then bound to the flexible spine of a protective cover of heavier, relatively inflexible material so that, when the opened front cover has received a massy enough stack of sheets, the book can lie flat. The technical term for this physical arrangement is codex (in the plural, codices). In the history of hand-held physical supports for extended written compositions or records, the codex replaces its immediate predecessor, the scroll.
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.
Although in ordinary academic parlance a monograph is understood to be a specialist academic work, rather than a reference work on a single scholarly subject, in library and information science monograph denotes more broadly any non-serial publication complete in one volume (book) or a finite number of volumes (even a novel like Proust's seven-volume In Search of Lost Time), in contrast to serial publications like a magazine, journal, or newspaper. An avid reader or collector of books or a book lover is a bibliophile or colloquially, "bookworm". A shop where books are bought and sold is a bookshop or bookstore. Books are also sold elsewhere. Books can also be borrowed from libraries. Google has estimated that as of 2010, approximately 130,000,000 distinct titles had been published. In some wealthier nations, the sale of printed books has decreased because of the use of e-books, though sales of e-books declined in the first half of 2015.