In 1984, "the book" is a pamphlet written by a party opponent and enemy of the state called Emmanuel Goldstein. Its official title is "The Theory and Practice of Oligarchical Collectivism" and Winston receives a copy of this book shortly before Hate Week begins, in Part 2, Chapter 9.
In essence, the book offers a scathing indictment of Big Brother and the party and seeks to explain the unequal distribution of wealth and power in Oceania. One example of this is Goldstein's explanation of the party slogan, War is Peace. According to the book, it is in the party's interest to maintain a constant state of war in Oceania. Specifically: "It eats up the surplus of consumable goods, and it helps to preserve the special mental atmosphere that a hierarchical society needs." In other words, instead of using the surplus of goods created by the people of Oceania for the benefit of everyone, the party redirects them to maintaining a constant state of war and keeping the people impoverished and controlled. We see this, for example, through the constant rationing of foods and the short supply of basic items, like shoelaces.
In this understanding, the word "war" does not signify a conflict between two countries; it is the state of living in Oceania in which the party battles to "keep the structure of society intact." There is no possibility of peace in Oceania because that would involve the dismantling of the party's system and, so, war really is peace. They are one and the same.
Secondly, the book addresses the nature of the party's power over the people of Oceania. Unlike other political parties and regimes, the party of 1984 will accept almost any person over the age of 16, and does not discriminate on any grounds. It does not have a capital city or figurehead, like a president or prime minister. It maintains its power by asserting its worldview through its language, Newspeak, and its institutions, like the Ministry of Truth, which form the fabric of society. Anybody who the party deems dangerous is executed by the Thought Police. Meanwhile, party members live under the constant surveillance of the telescreen and learn to adapt their thoughts and behaviours to espouse party views. In this respect, it is impossible to rebel against the party because the party is not a tangible thing; it is simply a worldview:
"The Party is not concerned with perpetuating its blood but with perpetuating itself. Who wields power is not important, provided that the hierarchical structure remains always the same."
The need to reproduce its worldview also explains the party slogan Ignorance is Slavery: by allowing the party to mold views and behaviors, people become ignorant of their own nature while simultaneously enabling the party to control them.
What is most important about the book is that it was, in fact, written by Inner Party member O'Brien. Goldstein is nothing more than a party invention. Goldstein and his book are designed to encourage Winston's rebellion and, ultimately, bring about his entrapment by the Thought Police and his incarceration at the Ministry of Love.