To be dreaming of being a ghostwriter can mean you are living life under a mask of another identity. This dream tells you have no confidence living life in your own identity. To dream of writing a bestseller means you know you have the ability to write well. You have to reconcile your anxiety of using your own identity, with your desire to produce good work.
How to use this dream t help you in waking life?
You might like to slowly build up your confidence. When you have a better self esteem, you will show your true face.