Thanks for the plug!  And thanks for reading a slightly earlier version.
My biography of Mark Twain (currently entitled "Mark Twain and the
Springs of Humor")  hasn't yet gone to press.  It is under contract to
be published by the University of California Press on the 100th
anniversary of Mark Twain's death (April, 2010).  I'm glad you mentioned
Robert D. Richardson, also the biographer of Emerson and last year
William James; he is one of my models for writing biography.

Jerry Loving