A book is a set or collection of written, printed, illustrated, or blank sheets, made of paper, parchment, or other material, usually fastened together to hinge at one side, and within protective covers. A single sheet within a book is called a leaf, and each side of a sheet is called a page. A book produced in electronic format is known as an e-book.

Book may also refer to a literary work, or a main division of such a work. In library and information science, a book is called a monograph, to distinguish it from serial periodicals such as magazines, journals or newspapers.

A lover of books is usually referred to as a bibliophile, a bibliophilist, or a philobiblist, or, more informally, a bookworm.