So i've read Mills Utilitarianism, Groundwork for the Metaphysics of Morals and Nichomachean ethics. What book on ethics should be next?
Critique of Practical Reason.
> Metaphysics of Morals
isn't complete without it.
Completely serious, you need Hume. He advocates for a 'sensible' sort of relativism where morality isn't objective, yet is relative to all of humankind. Very important ethical work.
Enquiry Concerning the Principles of Morals.
>nothing Plato wrote on Ethics isn't covered/bettered by Aristotle