Religion within the Limits of Reason Alone
I still remember the refreshing feeling I had when I first read Kant in my Philosophy class on Ethics at the University of Wisconsin. We had just finished reading the utilitarian ethics of John Stuart Mill and Jeremy Bentham, so the approach of Kant's categorical imperative seemed much more reasonable in comparison. That said, I am not now nor have I ever been a follower of Kant's ethics, but they are preferable to some ethical principles.
It was with this in mind, and a little reading in Kant since then, that I took up Religion Within the Limits of Reason Alone as part of a class on enlightenment literature at The Basic Program of the University of Chicago. This book, published beginning in 1791 - late in Kant's career, is also refreshing in its rational approach to morality. And while I am not convinced by his argument that it is morally reasonable to "act as if there be a God" I could follow his arguments for that approach to morality. The book provides an argument consistent with The Groundwork of the Metaphysics of Morals and is worth reading for all interested in ethics and Kant.
Religion Within the Limits of Reason Alone by Immanuel Kant. HarperCollins, New York. 2008 (1791)