Beck, L.,  A Commentary on Kant's Critique of Practical Reason, Chicago and London: University of Chicago Press, 1960.

From the publisher:

Few substantial philosophical treatises have been written in the same haste in which the Critique of Practical Reason was composed.  The book, however, shows a few of the signs of hurry that marred some of Kant’s other works. Indeed, the short time he devoted to thinking it out as a single self-contained book and to its actual writing may be responsible for a virtue singularly lacking in the Critique of Pure Reason,  which he meditated on for 12 years and “brought to completion in some 4 or 5 months,” presumably by making more or less judicious use of manuscripts composed at various times during the 12 year period.  Because, unlike the first Critique,  the second  adheres to one single straight line of argument, developed without exploring blind alleys and without getting almost lost on tangents. In its, we do not, as it were, listen to Kant thinking aloud or watch them trying first of this and then that way of making himself understood. The book has the magisterial tone and direct style your that comes to a man who has thought about all he wants to say before you put one word on paper.

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