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If you're not familiar with King's actual writings and (especially) speeches, you will not regret anything you do to rectify that. 50 years later, we tend to hold a pretty abstract notion of the man, as a kind of saint or—as Cornell West is fond of saying—a Santa-Claus figure. We are aware that he gave at least one important speech, possibly even more than one. But as you listen to these speeches, what you will become aware of is King's piercing intellect, and his devotion to the cause of uplifting the American nation and its people. You will see a man fully committed to nonviolence, aware of both its power and its difficulty. I was enraptured over and over again as I listened.
To be sure, this is not a biography. You will see few if any of King's faults and foibles as a human being here. The introductions, while well written and compelling, stick fairly closely to a vision of MLK as a moral and political leader. Which he certainly was. Feel free to look elsewhere if you are looking for a narrative of his life. But you could do worse than to start here if you are looking for an understanding of his spirit and his legacy.