A book series is a collection of related books that share the same characters, settings, or themes. Each book in the series usually tells a part of a larger story, allowing readers to follow the journey over multiple installments. Popular examples include the Harry Potter series by J.K. Rowling and the Percy Jackson series by Rick Riordan. Readers often enjoy book series because they can become deeply invested in the characters and their adventures. As the story unfolds across several books, it creates a richer and more immersive experience, making it exciting to see how the plot develops and resolves over time.