The Kentucky Rifle by Capt. John Dillin (pub. 1929, but reprinted many times)
Thoughts on the Kentucky Rifle in its Golden Age by Joe Kindig, Jr. (pub. 1960s)
American Rifle, a Biography by Alexander Rose (2009) This is a general information book, but a great read.
I ain't parting with mine, but these books will get you what you want to know about Kentucky Rifles.
-Buckles