There's just no way Sarah Palin wrote a 400 page book in just four months.
Of course a lot of people use ghost writers, but usually no questions are asked when it's clear that the "author" could, if time permitted, write a reasonably substantive and interesting book on their own. With Palin, it's obvious to anyone who's heard her original stuff that she could never write a book unassisted by a ghost writer -- she's simply not disciplined or bright enough to do it.