Head Honcho @ https://authorsunite.com/
I've helped launch over 100 books...most of them written by the actual author. A few have been ghost written.
I think it is always best to write it yourself because then the whole experience is way more fulfilling.
If you don't have enough time to do that, then you can definitely hirer a writer. The price range varies alot depending on amount of words and research time put in to compile all the content.
I have a team of writers if you are interested in getting a quote. Let me know :)
Good luck with your project!