A book series is a sequence of books having certain characteristics in common that are formally identified together as a group. Book series can be organized in different ways, such as written by the same author, or marketed as a group by their publisher.

Fictional series typically share a common setting, story arc, set of characters or timeline. They are common in genre fiction, particularly crime fiction, men's adventure and science fiction, as well as in children's literature.

Some works in a series can stand alone—they can be read in any order, as each book makes few, if any reference to past events, and the characters seldom, if ever, change. Many of these series books may be published in a numbered series. Examples of such series are works like The Hardy Boys, Nancy Drew, and Nick Carter.

Some series do have their characters go through changes, and make references to past events. Typically such series are published in the order of their internal chronology, so that the next book published follows the previous book. How much these changes matter will vary from series to series (and reader to reader). For some, it may be minor—characters might get engaged, change jobs, etc., but it does not affect the main storyline. Examples of this type include Tony Hillerman's Jim Chee and Joe Leaphorn books. In other series, the changes are major and the books must be read in order to be fully enjoyed. Examples of this type include the Harry Potter series.