What excerpts does Winston read from the book given to him by O’Brien? Why are these books of any significance?

Winston reads excerpts from The Theory and Practice of Oligarchical Collectivism, by Emmanuel Goldstein. The excerpts have to do with what Winston doesn't understand. He understands the how but not the why. In particular he is interested in the three slogans of the party, Ignorance is Strength, War is Peace, and Freedom is Slavery. These lead to many of the main themes in the book.