A letter from the Sell Collection
The letter at right is from the John M. Sell Civil War Collection. You might cite the item in your bibliography as follows:
Sell, John M., Letter to William Sell, 3 November 1861. John M. Sell Civil War Collection. Special Collections and Archives, University Library, California State University, Northridge. Los Angeles, CA.
See the Chicago Manual of Style, section 14.230, for more examples of citation formats in a Bibliography.