Only one client I ghostwrote a book for knew that he was in for a wrenching experience. That's because his father had a nervous breakdown when he was putting together the book he needed for tenure. All the others anticipated a happy intellectual journey, with great sales and applause at the end.
Books, every aspect of them but especially the actual creation, represent as formidable a process as our undergraduate education. Most of all, it's never what is expected. Essentially, that's because a book is an organic entity. As Chapter II is being created, the contents of the Introduction and Chapter I could and often will change. That's why I advise clients to get through the whole first draft before even thinking about revisions.
And, unless, a top publicist is hired, it's naive to dream about huge sales. Mostly the book will be distributed for nothing as a marketing tool. Still, even in this online video era, a print book is the price of entry for being taken seriously in most professions. Yet, the dream persists. Amazon.com sales are checked hourly. After a while there's that total mood deflation that there is no getting rich from the book.
The next time - and, yes, there is a next time - the client is prepared for several months of agony. Willingness to endure it comes for the success of the first book for not only getting business but increasing influence. A book might even have gotten them invited to TED Talks.