Brad,
Augusta State University in Augusta, GA, has the "Field Reports, Morning Reports, and orders for the Brookhaven Light Artillery" covering most of 1864. They are located in the George S. Storr Papers (1862-1864), Reese Library, Catalog #86-03, Folders 2-10 & 13.

Also, the Old Court House Museum in Vicksburg has copies of the letters written by Private Isaac E. Herring of Cowan's Battery. The letters date from 1861-1864.

I hope this helps,

Jeff