Here's what you need to know before you agree to be a “co-author” for a celebrity or "expert":
1. Your “collaborator,” no matter how famous, will have a lot less expertise than promised, and you will have to do a great deal of research for which you'll receive neither credit nor compensation.
2. Your collaborator will not understand what writing involves, or how long it takes, or that a second draft is not a final draft (never show your collaborator a first draft) or that reading a chapter and making suggestions is not the same as writing it in the first place.
3. You and your collaborator will both believe that the work—and hence the money—has been unfairly divided.
4. In short, an amicable divorce is easier to pull off than a happy collaboration.