Independent reading can occur both at school and at home. The books that students read should be somewhat easier for them to read with accuracy, while also having areas of literacy skills that need to be explicitly taught and practiced to mastery.
When children are "reading" books that are too hard, their reading engagement and time spent on-task and reading greatly decreases. Students do not understand key vocabulary and while many of our students can get a broad-stroke idea or gist of what is happening in the book, they do not understand what they are reading to depth that students need to effectively comprehend.
Authors write books to an intended audience and range of age. These audiences have had experiential knowledge that comes from being approximately the age of the targeted audience. As such, inferential material in these advanced books often go over a younger child's head, as they have yet to have such experiences in their lives. In short, these books are too complex for students to read on their own. They can make great Read-Aloud books, where parents read and discuss what their child cannot yet understand on their own.
Please see the section on Why Should Parents Read Aloud to Their Children? section of this website for more information on the benefits of reading aloud to their children.