Critique of Practical Reason and Other Works on the Theory of Ethics

Mary Gregor's translation is the gold standard in English and Andrews Reath's introduction is genuinely helpful — if you need Kant's Critique of Practical Reason, this Cambridge edition is the one to own. The catch is what it says on the tin: this is dense 18th-century academic philosophy, not self-help, and without prior Kant exposure most readers will bounce hard off the first chapter.