Hiring a ghostwriter can be a delicate procedure, especially when it comes to books. Time is a delicate factor and should be discussed at the time of hiring. A good ghostwriter can write a book in half the time required by someone else who is not a ghostwriter. Each writer will require a different span of time and level of involvement from the client. A ghostwriter may disappear after collecting data from you with little to no contact except for an occasional email or two. It does not mean that they're not a great writer. If you are the owner of a business that hogs all the time on your schedule, or maybe you're a single mother juggling between work and children, or any other reason why you can't spend much time or do not want to be involved in reviewing and editing that much, then you need to be hiring that kind of a ghostwriter.
You can find many great ghostwriters on freelancer websites but there is no way of ensuring if the writer will do justice to your project. Ideally, you want to find ghostwriters who have dedicated websites, or they're associated with an established agency to ensure the quality of work and authenticity expected from a writer in this business.
A good ghostwriter will have experience writing books in the same category, so they understand the niche and the do's and don’ts of the category whose book you want them to write. This is hard to express when the ghostwriter has signed non-disclosure agreements for all the books they've written. A good way to judge the skill level of a ghostwriter is to ask them for an outline at first. It can easily be judged at a glance how professional and experienced the writer is.