The letter is in the Provost Marshall Files held by Missouri State Archives. Its is indexed as being on microfilm reel F1622 and refers to a letter written by Maj JB Kaiser from Waynesville in Pulaski County Missouri on 9-23-1864.

http://www.sos.mo.gov/archives/provost/results.asp?txtName=Warthen&radSearch=ALL&txtKeyword=&selCounty=All

has the index entry for you. Instructions on getting info from the archives is on their resources home page at

http://www.sos.mo.gov/archives/resources/resources.asp#onlinedb

John R.