Final answer:
The subject of the question is a collection of letters written by different authors, which can be analyzed for their literary and historical significance.
Step-by-step explanation:
The subject of the question appears to be a letter or letters written by various authors. It includes examples such as Langston Hughes' letter to Ezra Pound, John Wheatley's letter to an English publisher, a soldier's letter to President Lincoln, and more. These letters provide insight into the thoughts, emotions, and experiences of the authors, and can be analyzed for their literary and historical significance.