Children's Books Meant For Adults

What exactly is meant as children's literature? Someone could write a book, label it suited for children, gear it toward children, even have children in the main plot or storyline and put it in the children's section of the bookstore. But there are many out there who would question whether some of these books are suited best for children or namely teens. One of them would be the wildly popular Harry Potter series. Now, if you have read the Potter books you know that it is writing that easily any adult would enjoy and understand, sometimes more so than children. While this is the less extreme of any examples there are plenty others that would be labeled children's books that should really be labeled as 'adults only' with their necromancers, high fantasy, gore, murder, magic, mystery and more.