During the third part of 1984, O'Brien states that he is in fact one of the authors of Goldstein's book (I think we can assume this to be true). I don't understand why the book was even created. At this point O'Brien is already well aware of Winston's anti-party sentiments, without needing to let him read a book that will add fuel to them.
Of course, it doesn't matter that the inner workings of the politic system are explained to Winston through the book - because he's going to be re-educated anyway, but I don't see why the book even needed to exist.
Interesting too that there's an exchange between Winston and O'Brien (I forget the exact wording) in which O'Brien says "I imagine you already knew most of what was in the book" and Winston agrees.
So, why does the book even exist?