These histories are free online. I’ll continue to update the list. There are other regimental histories that are modern and under copyright- those are not included. Feel free to contact me with any that are missing.
Connecticut: 1st Heavy Artillery, 2nd Heavy Artillery, 1st Light Battery, 5th, 6th, 7th, 9th, 13th, 14th, 15th, 16th, 18th, 20th, 21st, 25th, 27th, 29th
Delaware: 1st
Indiana: 3rd Cavalry, 7th Cavalry, Ninth Cavalry- 121st, 6th, 10th, 12th, 17th, 24th, 27th, 31st, 33rd, 36th, 37th , 38th, 42nd, 43rd, 44th, 46th, 51st, 52nd, 58th, 65th- Co E, 66th, 67th, 68th, 70th, 72nd, 73rd, 74th, 75th, 79th, 81st, 82nd, 83rd, 85th, 86th, 88th, 97th, 99th, 100th
Illinois: Battery A- 1st Light Artillery, Battery M- First Light Artillery, 2nd Cavalry- Co A, 4th Cavalry, 8th Cavalry, 9th Cavalry, 14th Cavalry, 17th Cavalry, 7th, 9th, 13th, 19th, 20th- Co K, 24th, 30th- Co A, 33rd, 34th, 36th, 39th, 40th, 45th, 46th- Jones, 46th- Woodbury, 47th, 50th, 52nd, 55th, 57th, 59th, 73rd, 77th, 84th, 85th, 86th, 92nd, 93rd, 95th, 96th, 102nd, 104th, 112th, 113th- Co B, 115th, 124th, 125th
Iowa: 1st Battery- Light Artillery, 1st Cavalry, 2nd Cavalry, 4th Cavalry, 3rd, 7th, 12th, 15th, 19th, 22nd- Volume 1, 22nd- Volume 2, 32nd, 33rd, 34th
Kansas: 7th Cavalry
Kentucky: 1st Cavalry
Maine: 1st Heavy Artillery, 1st Cavalry, 1st, 10th, 29th, 11th, 14th, 16th, 17th, 19th, 20th, 21st, 26th, 32nd
Massachusetts: 1st Battery- Light Artillery, 2nd Battery- Light Artillery, 5th Battery- Light Artillery, 9th Battery- Light Artillery, 10th Battery- Light Artillery, 1st Heavy Artillery (formerly 14th Infantry), 1st Cavalry, 3rd Cavalry, 1st, 2nd, 3rd, 5th, 5th Volunteer Militia, 6th, 7th, 9th, 10th, 11th, 12th, 13th, 15th, 17th, 19th- Waitt, 19th- Adams, 20th, 21st, 22nd, 23rd, 23rd- Co F, 24th- Volume 1, 24th- Volume 2, 25th, 25th- Co A, 27th, 29th, 32nd, 33rd, 34th, 35th, 36th, 37th, 38th, 39th, 42nd, 43rd, 44th, 44th in N.C., 44th- Letters, 45th, 46th, 48th, 49th, 50th, 52nd, 53rd, 54th, 55th, 57th, 58th, Minutemen of 1861, Independent Corps of Cadets
Michigan: Custer’s Cavalry Brigade, 2nd Cavalry, 7th Cavalry, 2nd Flags and Color Guard, 10th Cavalry, 4th, 9th, 20th, 24th
Minnesota: 1st, 2nd, 2nd- Drummer Boy’s Diary, 4th
Missouri: First Engineer Regiment and 25th Volunteers, 7th State Militia and Cavalry, 21st Infantry
New Hampshire: 1st, 2nd, 3rd, 5th, 6th, 7th, 8th, 9th, 11th, 12th, 13th, 14th, 15th- Volume 1, 15th- Volume 2, 16th, 17th, 18th, Surgeons
New Jersey: 1st Battery B Artillery, 1st Cavalry, 1st Brigade, 9th, 11th, 12th- Co F, 13th- Co D, 15th
New York: 4th Heavy Artillery, 9th Heavy Artillery, 10th Heavy Artillery, Duryee’s Brigade, 1st Cavalry, 5th Cavalry, 6th Cavalry, 8th Cavalry, 9th Cavalry, 10th Cavalry, 11th Cavalry, 15th Cavalry, 1st New York Dragoons (130th), 5th, 7th NY State Militia, 9th Zouaves, 10th Zouaves, 10th NY, 12th NY State Militia, 33rd, 34th, 40th, 44th, 48th, 49th, 60th, 72nd, 76th, 79th, 80th (20th State Militia), 81st, 83rd, 84th (14th Brooklyn), 93rd, 98th, 108th, 112th, 114th, 115th, 117th, 120th, 121st, 124th, 125th, 126th, 144th, 150th, 159th, 162nd, 165th, 179th, 184th
Ohio: 16th Battery- Light Artillery, 3rd Cavalry, 7th Cavalry, Fuller’s Ohio Brigade, 3rd Infantry, 4th, 5th, 6th, 7th, 8th, 11th, 15th, 20th, 25th, 26th- Co E, 29th, 32nd, 40th, 41st, 42nd, 46th, 47th, 48th, 53rd, 55th, 73rd, 78th, 81st, 81st- Co C, 83rd, 86th, 90th, 94th, 96th, 102nd, 104th, 105th, 122nd, 123rd, 124th, 125th, 126th
Pennsylvania: Durrell’s Battery, 3rd Cavalry, 4th Cavalry, 7th Cavalry, 8th Cavalry, 11th Cavalry, 15th Cavalry, 17th Cavalry, 18th Cavalry, Bucktails, 1st Reserves- Co K- 30th, 2nd Reserves- 31st, 3rd Reserves- 32nd, 12th Reserves- 41st, 22nd Cavalry and Ringgold Battalion, 1st, 7th, 11th, 23rd, 32nd State Militia, 45th, 48th, 49th, 50th, 51st, 52nd, 57th, 61st, 63rd, 69th, 72nd at Gettysburg, 77th, 78th, 83rd, 84th, 85th, 87th, 88th, 93rd, 95th, 97th, 101st, 101st- Co B, 102nd, 103rd, 104th, 105th, 106th, 109th, 111th, 116th, 118th, 121st, 122nd, 123rd, 124th, 125th, 126th, 127th, 131st, 132nd, 138th, 140th, 140th- Co K, 141st, 142nd- Reunion, 148th, 150th, 153rd, 155th, 155th- Co K, 187th, 192nd, 198th, Philadelphia Brigade, Reserve Corps
Rhode Island: Battery A- 1st Light Artillery, Battery D- 1st Artillery, 3rd Heavy Artillery, 5th Heavy Artillery, 5th Light Artillery, 14th Heavy Artillery, 1st Cavalry, 1st Cavalry at Middleburg, 1st Cavalry- Bliss, 1st Cavalry Bliss- Life with Sheridan, 1st Cavalry and 3rd Heavy Artillery- Chaplain Memoir, 1st, 1st- Co F, 2nd, 3rd, 4th, 7th, 9th and 10th and 10th Battery, 10th- High school boys of the regiment, 11th, 11th, 12th
Tennessee: 1st Cavalry, 13th Cavalry
U.S. Army: Berdan’s Sharpshooters, 5th US Cavalry
United States Colored Troops: 3rd USCT Cavalry, 33rd USCT (1st SC), 43rd USCT