C. S. Lewis Books with some relativity to morality:

Non-Fiction

The Abolition of Man

Mere Christianity

The Problem of Pain

Miracles

Fiction

The Screwtape Letters

The Great Divorce

Essays

(All Essays can be found in the book Essay Collection and Other Short Pieces not currently published in the United States. Check Amazon.co.uk for availability.)

Why I Am Not a Pacifist (see book The Weight of Glory)

Two Ways With the Self

Evil and God

Good Work and Good Works