I may be wrong about the name of the collection. I do know that Grady Howell referenced the same letters in his 3rd Miss. History.
The collection could be under her husband's name...I believe Saucier. Sally Carr corresponded with John Fairly of Co I 7th Miss. and Ed Carruth and others from the 3rd Miss.
Try manuscript collections at State Archives and USM for 3rd Regt. Miss. Inf., Surname Saucier and Carr.
I also would like to have any biographical information you have on E.B. Carruth and his brother James. They will be in the regimental history and I would like to have as much personal data as possible.
I just recently found the Parole Roster for the regiment as well as Sharp's Brigade Staff. Major Carruth is listed on Sharp's Staff.
Please contact me by email, but respond here as well.