What Winston seems most shocked by is the fact that O'Brien helped author the book. Then O'Brien futher tells Winston that the book is essentially a bunch of nonsense. Obviously this is used repeatedly to capture people like Winston and tell them the "truth"... or rather torture them to it.
Here is what O'Brien specifically says about the book:
The programme it sets forth is nonsense. The secret accumulation of knowledge -- a gradual spread of enlightenment -- ultimately a proletarian rebellion -- the overthrow of the Party. You foresaw yourself that that was what it would say. It is all nonsense.
Although O'Brien glosses over a series of ideas about a potential rebellion, it is as if Winston is learning that O'Brien knows what people will think, put it in there to pleasure them and get them to believe in something just to be able to catch the thinkers and then "clean them".
After Julia left O’Brien’s house, O’Brien informed Winston that it was important that he and Julia frequently alter their secret meeting venues. After this, he told Winston that he would be sending him a copy of Goldstein’s book which would take a couple of days to get hold of as they were few in number. He further informed Winston that the scarcity of the book was caused by the Thought Police's efforts to wipe it out of existence but that their efforts were futile as the book was indestructible because the Brotherhood had the capacity to reproduce the book almost precisely. O’Brien then instructed Winston on the procedure he would follow to get the book beginning with the misprinted word signal to the return of the book after two weeks.