One very useful source, a lengthy bibliography is _Mark Twain's German
Critical Reception: 1875-1986_ complied by J.C.B. Kinch
(Greenwood P, 1989).  Come to Elmira
in August and meet Carl Dolmech and thre's one
other guy whose name slips my mind.  I would
imagine the bib to _Our Famous Guest_ would also be helpful.