DIRTY LITTLE SEEKRIT DEPT.
Many, if not most, popular book series have actually been written by ghostwriters after the first one or two by the original authors achieve commercial success.
They send the ghostwriter(s) a plot outline or premise and a style sheet, and you crank it out and send it in and they pay you and it's edited and published under the famous writer's name.
This has been going on for centuries in almost all the arts, and is sometimes called the Studio System or Syndication.
Many famous works of art and music were and still are produced under this Studio System.
An artist or composer produces a few successful works, and either s/he or the publisher forms a "studio" to enhance output. And $.
For the example that first put me on to this, see...
http://www.theatlantic.com/entertainment/archive/2015/05/hardy-boys-nancy-drew-ghostwriters/394022/...although I became aware of this "system" looong before this particular article and was very disappointed because the Hardy Boys series was one of my favorite teen/pre-teen series of books.
Terry