The Ethics is the book I know best in the world having been working on my doctoral thesis on it before I left philosophy. It is not really an accessible work and even professional philosophers can need assistance to understand it. It is written in geometrical style and uses a lot of medieval and Cartesian terminology.
Its central thesis is that God and Nature are the same. It is a deeply rationalist work. It divides knowledge into three kinds, two of which are a priori in nature. It is first and foremost a work on Ethics. It is aimed at providing a good life and making humans free through understanding the world from Spinoza's position and by understanding how ones emotions can control you and getting control of those emotions through understanding. It in many ways holds positions that would more easily fit into Eastern traditions.
Pretty much the only thing that you need to know about it is that every single word of it is true.