In Part Two, Chapter Eight, of 1984, Winston and Julia go to O'Brien's apartment and discuss the underground resistance movement. It is in this scene that O'Brien tells Winston indirectly that Emmanuel Goldstein wrote the book, The Theory and Practice of Oligarchical Collectivism, known simply as "the book:"
"I shall send you a copy of THE BOOK..Goldstein's book, you understand, as soon as possible."
In Chapter Ten, however, O'Brien's true identity as a member of the Thought Police is revealed to Winston and Julia. O'Brien has them arrested and imprisoned in the Ministry of Love. It is here, in Part Three, Chapter Three, that O'Brien comes clean to Winston and confesses writing the book himself:
"I wrote it. That is to say, I collaborated in writing it."
This is an important development in 1984 for two reasons. Firstly, it proves that all previous conversations with O'Brien were just a ruse to lure Winston into openly committing thoughtcrime. Secondly, it demonstrates one of the many ways in which the party is able to control its citizens. As O'Brien says, the party has created Goldstein and his book so that it can maintain absolute power over Oceania:
"The heretic, the enemy of society will always be there, so that he can be defeated and humiliated all over again."
In sum, then, O'Brien is the true author of Goldstein's book and it was written to enable the party to sniff out rebellion amongst its members and to punish it severely.