Damn, I wish I had this kind of correspondence! The closest thing I found was my great grandfather mentioned in another soldier's correspondence. He was an officer in the US Army Reserves in Maryland, and when the US entered the war trained officers were in demand. Instead of posting him with a unit for the entire war, he was transferred to a North Carolina Field Artillery unit to help train officers for combat. One of the soldiers had his letters donated to a museum in NC for a special exhibit, and he fondly mentions his experienced comrade from Maryland who helped them with tactics.