It seems that there are no other links on the internet to information about this regiment. I am posting some brief information from Confederate Military History and a bibliography on this regiment. I do this in honor and remembrance of the men who served with this regiment, lest they be forgotten.
The Twenty-first regiment Georgia volunteers was organized with John T. Mercer as colonel, James J. Morrison, lieutenant-colonel, and T. W. Hooper, major. The adjutant was T. J. Verdery. The captains were T. C. Glover (A), A. S. Hamilton (B), J. F. Waddell (c), H. T. Battle (D), J. R. Hart (E), John T. Boykin (F), Wesley Kinman (G), James C. Nisbet (H), Michael Lynch (I), John B. Ackridge (K).. The Twenty-first served in the army of Northern Virginia, acting a gallant part in the many great battles in which it was engaged. Col. John T. Moore being killed in battle, was succeeded by Thomas W. Hooper. Lieutenant-Colonel Morrison was followed by Hooper; T. C. Glover, who had succeeded Hooper as major on his first promotion, became lieutenant-colonel when Hooper was promoted to the command of the regiment. When Glover became lieutenant-colonel, M. Lynch became major. Adjutant Verdery was followed by L. F. Bakewell. Captain Glover was followed by W. M. Butt, who was killed in battle. Captain Kinman was succeeded by N. B. Hudgins, and Nisbet by John B. Countiss. The Twenty-first was one of the regiments commanded by the gallant General Doles, who fell at the second battle of Cold Harbor.
Company Organization of the 21st Regiment.
Company A: Campbell County men-Campbell County Guards/Campbellton Home Guards
Company B: Floyd County men-Floyd Sharpshooters
Company C: Fulton County men-Atlanta Volunteers
Company D: Polk County men-Cedartown Guards
Company E: 1st Company E-Floyd County men-Sardis Volunteers. By special order no. 106, Adjutant and Inspector General's Office, dated Richmond, Virginia May 8, 1862, this company, "old Company E," was detached from this regiment and became Company A, 1st Regiment Georgia Partisan Rangers and subsequently became Company G, Smith's Legion Georgia Cavalry, which in March 1863, became Company G, 6th Regiment Georgia Cavalry.
Company E: 2nd Company E-Forsyth and Dawson County men-Concord Rangers. This company was originally known as Captain Smith's Company Independent Regiment, Wise's Legion. It was formed of men transferred from Captain Jesse Burtz's Company, the Cherokee Georgia Mountaineers, which was divided by Special Order No. 163, Adjutant and Inspector General's Office, dated September 26, 1861. It afterwards became Company D, 2nd Battallion, North Carolina Infantry, but being composed of men from Georgia was subsequently transferred to the 21st Georgia Infantry regiment by Special Order No. 84, Adjutant and Inspector General's Office, April 11, 1864 , and became the new Company E.
Company F: Troup County men-Ben Hill Infantry-Ben Hill Volunteers
Company G:Gordon County men-Dabney Rifles
Company H: Dade County men-Silver Grays/Yancey Invincibles
Company I: Stewart County men-Stewart Infantry
Company K: Chattooga County men-Bartow Avengers
Bibliography- 21st Georgia Infantry
Confederate Military History, Extended Edition. Vol. 7: Georgia. Wilmington, NC: Broadfoot, 1987. E484C65.1987v7. See p. 42 for a brief unit history.Crute, Joseph H., Jr. Units of the Confederate States Army. Midlothian, VA: Derwent Books, 1987. Ref. See pp. 98-99 for a concise summary of the regiment's service.
Georgia State Division of Confederate Pensions and Records. Roster of the Confederate Soldiers of Georgia, l86l-l865. Vol. 2. Hapeville, GA: Longino & Porter, 1959. pp. 838-934. Unit roster.
Jones, Charles E. Georgia in the War, 1861-1866. Atlanta, GA: Foot & Davies, 1909. See p. 27 for an incomplete list of unit officers.
Nisbet, James Cooper. Four Years on the Firing Line. Chattanooga. Imperial Press. 1915, 445 pgs.
Sifakis, Stewart. Compendium of the Confederate Armies:...Georgia. NY: Facts on File, 1995. pp. 224-25 (2 pages). E577S53.1995.
Thomas, Henry W. History of the Doles-Cook Brigade. 1903.
Wiley, Bell I., ed. 4 Years on the Firing Line. Jackson, TN: McCowat-Mercer, 1963.