My dear friend Lindsy Lawrence puts it best:
[“Letter from Birmingham Jail”] has all the power of the pulpit, but that power is tempered into a controlled, but passionate argument for equality and how continuing to accept inequality destroys not only the fabric of our social contract but also our human connections. I consider it one of the key documents for understanding what it means to be a US citizen. It’s just as important as the Federalist papers or Lincoln’s “Gettysburg Address” in defining how we should come to see ourselves in this country.
The essay can be found here. For my druthers, King’s best piece ever was his last, the “I’ve Been to the Mountaintop” speech delivered the day before his assassination. Together, these two pieces are perhaps the clearest declarations of what America can mean, and how far we’ve got to go before we get there.