I am reading "The Oxford Companion to Mark Twain" and enjoying it immensely.
I have found one error that I'd like to point out, though. On page 408 it states that Jackass Gulch is in Calaveras County. It is not; it is in neighboring Tuolumne County.
I lived in Calaveras County for 29 years (including a couple of years in Angels Camp). My wife was born in Tuolumne County. Jackass Gulch, or Jackass Hill, where the reconstructed cabin where Twain stayed with Jim Gilless in the winter of 1864 and 1865 is located, is near the Calaveras County line, but on the Tuolumne side.
BTW, Twain mentions a "Tuttletown, Arkansas" in one of his writings. There is no such place, as far as I can tell. There is a Tuttletown, California, though - in Tuolumne County, not far from Jackass Hill. - B. Clay Shannon