Terry, I would have to say that in my educated opinion there is not a MSM regiment that is better documented than the 3rd MSM Cavalry regiment.
Regarding the 3rd MSM I have four crates of materials that I have organized, three crates of materials that I have yet to organize, and a couple of archival sources that have large reservoirs of material that I have barely tapped into.
There is a myth that has been bandied about the southeast Missouri historical community that there are no materials on the 3rd MSM, but that is all it is--a myth. One single writer propagated it back in the 1980s and 90s, and for some reason it has had a paralyzing effect on subsequent researchers of the regiment. I have found that when dealing with southeast Missouri Civil War matters that it is best not to bet the farm on just one person.
The fact of the matter is that U.S. Senator John B. Henderson--the author of the 13th Amendment that freed the slaves--and U.S. Attorney General Edward Bates were initial and primary patrons of the 3rd MSM Cavalry regiment. Henderson and Bates were both close to Lincoln, and, with the exception of Frank Blair, were the most prominent Missourians in Washington D.C. during the war. Both Henderson and Bates had close relatives who were senior commanding officers of the 3rd MSM. The 3rd MSM, beginning with its predecessor Home Guard organization, was on duty from before Fort Sumter until after Appomattox. The 3rd MSM operated in more areas of Missouri than any other U.S. regiment, MSM or Volunteer. The 3rd MSM is the only MSM regiment that was involved in an engagement as a complete regiment. During Marmaduke’s Second Raid, the 3rd MSM was the first U.S. unit to engage the Confederates and afterward participated in every engagement of the raid. During Price’s Raid the 3rd MSM was the first U.S unit to engage the Confederates, and afterwards participated in more engagements than any other U.S. unit during the raid.
On such organizations there are vast amounts of original material.