A ghost writer is someone who writes a book which is then published under another name. Sometimes this is done under a fake collective name like the name Franklin W Dixon for The Hardy Boys, which was actually written by many different authors. Other times, like in Animorphs, it's when multiple writers take over a series later on while the books continue to be published under the original authors name. Most of the books starting in the late 20's are written by people other than KA, even though they were published with her name as the author. To know who ghost wrote the book, look for her dedication at the beginning of them, which is usually dedicated to her son and to the actual author.