A book is a written work of substantial length created by one or more authors. They can be distributed in various forms such as printed books, audiobooks, and electronic books (ebooks). Books are broadly classified into fiction, which contains invented narratives or other imaginary content, and non-fiction, which contains content intended as factual truth.

The term may also refer to the physical or electronic object containing such a work. Modern books are typically printed in a codex format, composed of many pages bound together and protected by a cover. Before the wide adoption of the modern printing press, codices were used to create handwritten manuscripts. Older writing media include scrolls and clay tablets.

The book publishing process is the series of steps involved in its creation and dissemination, often undertaken in modern times by a commercial publishing company. The publishing industry has recently seen major changes due to new technologies, including ebooks and audiobooks (recordings of books read aloud). Awareness of the needs of people with print disabilities has led to a rise in accessible publishing formats such as braille printing and large-print editions. Google Books estimated in 2010 that roughly 130 million total unique books have been published.

Books are sold at general retail stores and specialized bookstores, as well as online, and can be borrowed from libraries or public bookcases. The reception of books has led to several social consequences, including censorship.

Books are sometimes contrasted with periodical literature, such as newspapers or magazines, for which new editions are published according to a regular schedule. Some objects broadly described as "books" are left empty for personal use, such as notebooks, diaries, sketchbooks, account books, and autograph books.