A chapter book or chapterbook is a story book intended for intermediate readers, generally age 7-10. Unlike picture books for beginning readers, a chapter book tells the story primarily through prose, rather than pictures. Unlike books for advanced readers, chapter books contain plentiful illustrations.
Many children who have learned to read are just beginning to experience the 'joy of reading' on their own. Beginning readers who want to read independently often look for books that reflect the developmental needs of their age.
It's an adventurous age where children are keen to do things on their own and are starting to understand and accept others' viewpoints. They feel more grown-up and want the books they read to reflect this. They don't want life or stories to be too complicated, but they do like to push the boundaries.
