A book is a set of sheets of paper, parchment, or similar materials that are fastened together to hinge at one side. A single sheet within a book is a leaf, and each side of a leaf is a page. Writing or images can be printed or drawn on a book's pages. An electronic image that is formatted to resemble a book on a computer screen, smartphone or e-reader device is known as an electronic book or e-book.

The term "books" may also refer to a body of works of literature, or a main division of literature (e.g., children's literature) . In library and information science, a book is called a monograph, to distinguish it from serial periodicals such as magazines, journals, or newspapers. In novels and sometimes other types of books (for example, biographies), a book may be divided into several large sections, also called books (Book 1, Book 2, Book 3, and so on). 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 in some department stores, drugstores and newspaper vendors. Books can also be borrowed from libraries. Google has estimated that as of 2010, approximately 130,000,000 distinct titles had been published.[1] In some wealthier nations, the sale of printed books has decreased because of the use of e-books,[2] though sales of e-books declined in the first half of 2015.[3]