I've given two papers on the subject, one at Elmira and the other at
ASA.  The strongest arguments are contained in the letters from Susy to
Louise Brownell.  Some of the letters are to be found in Charles Neider's
Introduction to "Papa," and photocopies of Susy's letters to Louise are in
the Mark Twain Papers.  The originals are in the library of Hamilton
College.
There is no question but that it was a passionate relationship, a love
relationship--but I am cautious about the language I use to describe it.
Does it explain Susy's sudden removal from Bryn Mawr?  My guess is yes--the
school year was not complete and there was no other reason that I am aware
of that would have required Susy to withdraw before the year was
over.  From mutual friends we also know that Louise had asked Susy to live
with her the following year at Bryn Mawr.
There's more to be said on this subject, which I intend to do one day
soon....

Linda Morris