Bryce, the IL State Historical Library lists the following ms sources on the 17th Il Cavalry:

Cyrus Hutchinson papers, including official correspondence and "reminiscences", SC 732. Includes official correspondence of William H. Austin, Henry Christian, John A. Hynes, James S. Upham
Charles G. Robbins letter, 5-11-65. SC 1212, in Reuben T. Prentice Papers.

Try Cheryl Schnirring at the Abraham Lincoln Presidential Museum and Library, Cheryl-Schnirring@ihpa.state.il.us