
Immanuel Kant's Groundwork of the Metaphysics of Morals ranks alongside Plato's Republic and Aristotle's Nicomachean Ethics as one of the most profound and influential works in moral philosophy ever written. In Kant's own words its aim is to search for and establish the supreme principle of morality, the categorical imperative. Kant argues that every human being is an end in himself or herself, never to be used as a means by others, and that moral obligation is an expression of the human capacity for autonomy or self-government. This edition presents the acclaimed translation of the text by Mary Gregor, together with an introduction by Christine M. Korsgaard that examines and explains Kant's argument.
The deepest and most interesting philosophical books are generally difficult and best accompanied by some form of instruction. This is true regardless of how clearly (e.g. Plato) or esoterically (e.g. Kant) written they are. There are some good secondary sources that can help but it's far preferable to be able to talk to an expert in a one-on-one or smallish setting. It would take a long time to explain why this is the case, so I'll just leave it as an assertion here.
Okay, that being said, I wouldn't want to dissuade anyone from reading philosophy on their own. Here are some recommendations for books and articles that can be (more) fruitfully read on one's own. Anything marked with a + is a secondary source. Ideally I'd be giving you excerpts from each of these in a reader for a course. I'm going to skip some people like Aristotle who should be approached in carefully selected excerpts or with a guide, and also some more obvious things that almost everyone with a college education has read, like Descartes' Meditations or Plato's Republic.
On Fate, Alexander of Aphrodisias
+ Problems from Locke, Mackie
Tractatus Theologico-Politicus, Spinoza
+ The Courtier and the Heretic, Stewart (on Spinoza and Leibniz)
Enquiry Concerning Human Understanding, Hume
Groundwork of the Metaphysics of Morals, Kant
Prolegomena to Any Future Metaphysics, Kant
On Liberty, Mill
"Modern Moral Philosophy", Anscombe
"What is Capitalism?", Rand
The Bounds of Sense, Strawson
+ "Rawls on Justice", Nagel
"The Naturalists Return", Kitcher
Sources of Normativity, Korsgaard
This crowd in particular might be interested in some of Frege's work as well, e.g. "What is a Function?" and "Sense and Reference."
The author seems to be blissfully unaware of existentialism, French or otherwise, which could have either saved the author 12 years of anguish (OTOH, a very existentialist thing to have) or made the article more interesting. Maybe both. This way I'd say, needs work.
He takes it as axiomatic that we cannot use empiricism to determine universal rules, which is something that seemed pretty well established as false by the time between when it was written and when I read it.