Groundwork of the Metaphysics of Morals by Immanuel Kant

Act only on that maxim you could will to become a universal law. Kant's test for right action is stated here, and defended more rigorously than anywhere else.

He argues that morality cannot rest on consequences or feelings but only on duty and rational consistency, and derives the requirement to treat every person as an end and never merely as a means.

Short, difficult and one of the two or three most important works of moral philosophy ever written.