A book is a series of pages assembled for easy portability and reading, as well as the composition contained in it. The book 's most common modern form is that of a codex volume consisting of rectangular paper pages bound on one side, with a heavier cover and spine, so that it can fan open for reading. Books have taken other forms, such as scrolls, leaves on a string, or strips tied together; and the pages have been of parchment, vellum, papyrus, bamboo slips, palm leaves, silk, wood, and other materials.
The contents of books are also called books, as are other compositions of that length. For instance, Aristotle's Physics, the constituent sections of the Bible, and even the EgyptianBook of the Dead are called books independently of their physical form. Conversely, some long literary compositions are divided into books of varying sizes, which typically do not correspond to physically bound units. This tradition derives from ancient scroll formats, where long works needed several scrolls. Where very long books in codex format still need to be physically divided, the term volume is now normally used. Books may be distributed in electronic form as e-books and other formats. A UNESCO conference in 1964 attempted to define a book for library purposes as "a non-periodical printed publication of at least forty- nine pages, exclusive of cover pages".