He began his letter by explaining why he, a preacher from Atlanta, was compelled to be in Birmingham. It drew to a conclusion with this:
Never before have I written so long a letter. I'm afraid it is much too long to take your precious time. I can assure you that it would have been much shorter if I had been writing from a comfortable desk, but what else can one do when he is alone in a narrow jail cell, other than write long letters, think long thoughts and pray long prayers?
Dr. King's long thoughts inspired thought in others, and the struggle for freedom and equality moved forward. Sadly, the struggle continues to this day.
If you have a reading device look for Gospel of Freedom by Jonathan Rieder [Overdrive ebook], which provides background and analysis of the letter.