For MLK Day, I usually recommend Letter from a Birmingham Jail and something else. This year, I recommend to myself that I reread Strength to Love, since I read it almost 15 years ago with considerably less life experience and considerably less settled theological views.
The 'something else' is Ralph Ellison's Invisible Man. Though not the work of a day of reading, it is, very possibly, the Great American Novel. It takes the very American theme* that people, on the whole and individually, are squalid, or crass, or manipulative, or ignorant, or mob-like, or unwilling to stand up for themselves, or unwilling to stand up for anyone else, and somehow concludes that these people might be worth loving, caring about, fighting for and alongside anyway.
I think we tend to want as allies only those people who are purer than pure, and look askance at people with obvious agendas or personal problems. The tendency to run down MLK because of his treatment of his wife, or his doctoral dissertation, or to think of him as in some way 'less smart' than Malcolm X, is a perfect example of this. It is somehow not enough that he be charismatic, visionary, a good speaker and a man who knew how to get important results. We would do well to remember the point that my old dissertation subject, Hugo Grotius, made. Human motivations are multifarious and almost never pure, but this is no discredit to them, since to count it so would be to discredit every last political and social cause. Instead, we ask people to --and judge them by their willingness to--uphold the moral principles they espouse even as we all disagree. And their are worse moral principles to uphold than "I will treat you like a human being even if you don't always treat me like one."
It seems to be the lot of reflections on race in America that they remain ever relevant, and in some ways Ellison's is the most important note to strike.
*see also: Huck Finn, To Kill a Mockingbird, It's a Wonderful Life