A book is a set of printed sheets of paper held together between two covers. The sheets of paper in a book are called pages. The pages have words written in them and illustrations drawn. The book is a more flexible format than the earlier idea of the scroll. The change from scrolls to books began in the Roman Empire and took many centuries to become complete. A writer of a book is called an author. Someone who draws the pictures in a book is called an illustrator. Books can have more than one author or illustrator. A book can be a text that is a part of a larger collection of texts. A section of a text may be published as a book so that it only has one author or only focus on one subject area. Books written in this way can be understood without reading whole collection of writings. Examples are the Iliad, Odyssey, Bible, Quran and Torah. All of them consist of a number of “religious books” in this sense of the word. Encyclopedias often have separate articles written by different people, and are published as separate volumes. Each volume is a book. Hardcover books have hard covers made of cardboard that is covered in cloth or leather and sewn together. Paperback books have covers made of stiff paper that is glued together.