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.

Eoin Colfer ( /ˈoʊ.ɨn/ OH-in; born 14 May 1965) is an Irish author. He is most famous as the author of the Artemis Fowl series, but he has also written other successful books. His novels have been compared to the works of J. K. Rowling. In September 2008, Colfer was commissioned to write the sixth instalment of the Hitchhiker's Guide to the Galaxy series, entitled And Another Thing....

Eoin Colfer was born in Wexford, Ireland. He attained worldwide notice in 2001, when his first Artemis Fowl book was published. He then became a New York Times best-selling author of books for children and teens, known for writing the Artemis Fowl series (which consists so far of 7 books that were all New York Times best sellers), Half Moon Investigations, The Wish List, and The Supernaturalist, along with the Eoin Colfer’s The Legend of... books. In January 2008, Colfer published a book titled Airman, which was also a New York Times best-seller. To date the majority of his works have reached the New York Times Bestseller list at least once. Colfer currently resides in Wexford, Ireland with his wife, Jackie, and two children, Séan and Finn.