A book series consists of multiple books that are connected through shared characters, themes, or narrative arcs. These books often follow a particular storyline over several installments, allowing readers to immerse themselves in an expanding universe. For children, a series can provide a sense of familiarity and attachment to characters, making reading more engaging and encouraging a habit of reading. Popular children's book series include "Harry Potter," "The Magic Tree House," and "Percy Jackson," each offering adventures that build upon previous stories.