Gregor, Mary, Laws of Freedom: A Study of Kant's Method of Applying the Categorical Imperative in the Metaphysik der Sitten, Oxford: Basil Blackwell, 1963.

From a review:

“The English speaking world waited 165 years for an adequate translation and a full study of Kant’s Metaphysik der Sitten,  and then in the course of one year got to translations of its long Introduction and Doctrine of Virtue as well as this fine commentary. One of Kant’s most interesting and difficult works is now finally available.”

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