I can't help but reflect on the likelihood that Dr. King modeled his title after Oscar Wilde's celebrated "Letter from Reading Gaol." (Gaol is pronounced jail and is a nearly archaic alternative spelling.)
Wilde was imprisoned for being gay, and his cri de coeur is a heartbreaking read. Dr. King's letter is very different, of course, but he must have known people would see see and think about how close the titles are.
He must have been thinking quite a lot about the oppression of human minorities.
I love his stirring words, especially when he urges moderates to stop being part of the problem.
Many of us today need to hear that and think hard on it!