A book is as long as it needs to be.
Many ghostwriters and companies try to limit the length of a book. Their fee is often calibrated for a 200-page manuscript, which is extremely short considering that the printed version is likely to run 150 to 175 pages. Many sales reps even tell prospective clients that 200 pages is now the industry standard in publishing. That's nonsense.
Trying to limit the length of your book not only compromises your ideas, but it maximizes the profit of a ghostwriter or company. Many ghosts and companies, for example, offer to write a memoir of 200 pages for $30,000 over a six-month period. This means that a writer only has to write a little more than one page a day! Do you really want to pay $30,000 for that kind of work ethic?
Your idea or book deserves to be judged on its merits, not on an arbitrary length related to a fee structure. What if someone had told Shakespeare "Sorry, Will, but Hamlet is running too long. Cut Act III"?
Whether the book is your memoir, a nonfiction book on earth-crossing asteroids, or an action-packed thriller, don't let anyone convince you to shorten it just because it will cost less. Telling someone to shorten a book is one of the biggest scams in online ghostwriting – and there are many more.
I encourage you to download your free copy of How to Hire a Ghostwriter. from this site.
You may also want to read How Books Are Written.