Thank you everyone who's provided their insight on the topic. Is
Michelle Cotton's thesis readily available through interlibrary
procedures? My own exploration on the topic has achieved similar
results as those expressed here: Hamilton College's archives hold
roughly forty letters from Susy to Louise Brownell. Brownell later
married A.P. Saunders, a chemistry professor and dean of Hamilton
(the archivist has lead me to believe that is how they acquired the
letters). The archive is in the long-term process of digitizing their
holdings as we speak.

My curiosity concerning the Brownell correspondence involves
something that Twain actually published.  In his "In Defense of
Harriet Shelley" essay (first published in the North American Review
of July, August, and September 1894) he twice mentions "girls'
colleges of America." In May I received my MA in editorial studies
with a critical edition of the two manuscripts that comprise Twain's
work.  My research has led me to believe that part of Twain's
interest in writing the essay was to discount Brownell academically.

The evidence pointing towards this conclusion is not circumstantial,
but to be fair, Brownell's letters could stamp it certified. In any
event, should any specialist of Twain's later writings be interested
in my MA, I would be thrilled at further constructive criticism.

Thank you!
Alex Effgen
Boston University