I've just learned of a pretty obscure online text about Twain's
Civil War experiences that I don't find cited in any of the Twain
biographies on my shelf:

Absalom C. Grimes, "Campaigning with Mark Twain"
http://www.chickasaw.com/~rainbow/soc/archives1.htm

This is a chapter from Grimes's memoirs, _Confederate Mail Runner_
(New Haven: Yale University Press, 1926), and is online at the
Soc.History.War.US-Civil-War Archives.

Grimes figures prominently in both Twain's "A Private History of a
Campaign That Failed" and the chapter on the war in Paine's
biography.  Paine also mentions Grimes as someone who was still alive
to provide him with information about that period in Twain's life.
Grimes died in 1911 so Paine might have had access to this
written account in manuscript form.  Leah Berkowitz, who alerted me
to the text, says that the manuscript for Grimes's book is in the
Burton Historical Collection of the Detroit Public Library.

Jim Zwick