Clark, W., & Clark, G. R. (1806). [Letter to his brother, George Rogers Clark: Dated at St. Louis, September 23, 1806 : and containing a summary of the Lewis and Clark expedition]. [Evening Fire-side?].
Chicago Style (17th ed.) CitationClark, William, and George Rogers Clark. [Letter to His Brother, George Rogers Clark: Dated at St. Louis, September 23, 1806 : And Containing a Summary of the Lewis and Clark Expedition]. [Place of publication not identified]: [Evening Fire-side?], 1806.
MLA (9th ed.) CitationClark, William, and George Rogers Clark. [Letter to His Brother, George Rogers Clark: Dated at St. Louis, September 23, 1806 : And Containing a Summary of the Lewis and Clark Expedition]. [Evening Fire-side?], 1806.
Warning: These citations may not always be 100% accurate.