First, check Tom Tenney's extensive Mark Twain: A Feference Guide. (G.K. Hall). Then check Tom's supplements to it in American Literary Readism, I think from 1977 to 83. He has also added much in most of the Mark Twain Circular's "About Mark Twain" section at the back of many issues. For easy use of all this, consult my index of all this here on the Forum or in various issues of the Circular.