What is the book Winston reads about in "1984"? What does he realize after? Goldstein's The Theory and Practice of Oligarchical Collectivism is...

In the
dystopian world of Big Brother, thought-control, Newspeak, Doublespeak, and reversible and
constant wars, there is a perpetual shifting of what is considered real; or, in the words from
Shakespeare's phantasmagoric play, Macbeth, "Nothing is what is
not." Further in this oxymoronic existence, afterandbegin to conduct their affair in a room
above Mr. Charrington's shop, at The Ministry of Truth, the impostor  approaches Winston and
asks him if he would be interested in joining the Brotherhood; moreover, he offers Winston
Goldstein's book that contains strategies on how to destroy Big Brother. In truth, however,
O'Brien is actually a member of the Inner Party that supports the government of Big
Brother. 

Then, at the end of Chapter IX of Book Two, after reading
Goldstein's book, Winston realizes that he is not mad, even if he is "a minority of
one":

There was truth and there was untruth, and if
you clung to the truth even against the whole world, you were not mad.

This is the difference between Winston at this point and Macbeth,
who, in his madness,  believes in what the witches from the preternatural world have told him.
Sadly, then, Winston also succumbs to madness when, after torture in Room 101 where his greatest
fear attacks his psyche, he goes mad and agrees that two plus two is five, the very argument he
has so long resisted.