Spinoza's Purpose in Writing The Ethics

In another treatise On the Emendation of the Intellect, Spinoza writes of his desire to discover a way to achieve "continuous, supreme and unending joy." It does look like this is related to The Ethics as the work begins with a view of infinite reality and ends with human freedom, but is that where Spinoza takes himself and us along with him?

Augustine comments in The Confessions that "all humans want to be happy" in a propositional sounding phrase that would please Spinoza. Spinoza spends significant chapters of the emotions of sadness, joy and desire and encourages a pleasant life, but the longer I live with this work, the less it appears to be focused on being connected with the highest happiness. If not, what is it?

First of all, Spinoza spends significant time discussing God's expressiveness. God expresses through modes, of which we are each one. This expressiveness is not teleological, that is, God does not express for achievement of some goal. Rather the expressiveness is the manifestation of potential power which necessarily optimizes because God necessarily optimizes. 

As a modal expression of God's power, it would appear that each of us is expressive and that our expressiveness is a function of power. For that reason, health, education and social resources allow fuller expression of our modality - a virtual self becoming actual through interactions. 

It would appear that Spinoza's attributal division of extending and thinking means that our mind registers joy as the freedom to fully express ourselves, even when our mind is clouded by delusions of imagination to see ourselves as powerful. Certainly such a state seems unlikely to be what Spinoza is discussion. Instead reality and perfection are the same thing so as we move to reality, we also move to perfection and greater sustainable joy. 

Does that mean that great scientists have greater joy? I think Spinoza might reply that it depends on the fullest expression of self, which might entail notions of greater health or wealth. Instead, as I continue to explore this work, I would hazard that the ability to be fully present, without the delusions of imagination, by good health, reason and resources, represents the point of optimal modal expression of some virtual becoming actual and that optimal modal expression is the purpose.