What are the book and the brotherhood?

Goldstein's book is The Theory and Practice of Oligarchical Collectivism. It is sort of a Bible of revolutionary thought meant to destabilize Oceanea and break the Inner Party. We are not sure that Goldstein is even real. The book is a fabrication written in part by O'Brian himself. The Party needs an enemy and the book validates the enemy. The "brotherhood" is a secret underground organization meant to bring about revolution. I don't think they exist.