the book

[ClassicRockKid]

why does O'Brien give winston the book that he wrote himself if they already know that he is against Big Brother?

[The Atheist]

why does O'Brien give winston the book that he wrote himself if they already know that he is against Big Brother?

Two things:

It enables Orwell to peddle his brand of anti-totalitarianism even more specifically. He uses a similar device in Homage to Catalonia where he includes a political commentary which is an appendix to the story.

It enables O'Brien and the Thought Police to study W & J's reaction to it. Given the psychology and brainwashing, it might have proved useful to them.