Choosing a ghostwriter is such a big deal. This is a person who is going to write your words for you, so you want to be very careful with how you choose the person. First, make sure that the person has a similar style to your own. You do not want someone writing in an extremely formal if you are aiming for a casual approach to the book.
Second, and this is really important, make sure the person is a great communicator. Spend some time talking to and with this writer, on Skype, on the phone, on Yahoo Instant Messenger, and see if you are connecting with each other. See if this person has the same excitement about the book, the same enthusiasm as you do about the project. You will be really disappointed and frustrated if the person communicates in a different manner.
Another thing to look at is writing samples. You want to make sure this persons writing style is comparable to your own preferences. You also want to make sure this person can write and can write efficiently. Some writers excel in certain fields but lack skill in other fields, so you have to make sure the writing samples fit into your e-Book and your expectations of how it will be published.
Next, you need to make sure that the ghostwriter has a good reputation. I have to be careful here and explain something. One bad review, or two, do not mean that the writer is bad necessarily. Sometimes, miscommunications happen, and misunderstanding are inevitable sometimes. As long as the person has an overall positive review set, they are probably trustworthy. A lot of this is trial and error, too. You will soon develop a good hunch about writers and workers in general. You will talk to someone and think, Hmm. He or she seems to be good for writing this book, or, Nope, this is a really bad match.
One more thing, and I think this is oftentimes forgotten, is you have to let them know that they are being ghostwriters. They need to know that they will not necessarily get credit for their writing and some people will not like that they will not get credit. They also need to be okay with the idea of ghostwriting. It is a really different concept to writing usually. So, you would have to be very clear about that with the worker prior to the beginning of the project. Be clear, be open and honest, with the worker, and nothing but good things will come your way. Good luck with your book. Happy ghostwriting and editing. It will be a blast!