I believe that the photographer you are looking for is E.O. Hoppe'
(rather than E.O. Hopper).  A musician by the name of Michael Hoppe' has
recorded a couple of very nice CD albums on the Bainbridge label entitled
"The Yearning" and "The Dreamer".  The liner notes include some fabulous
photographs taken by his grandfather (E.O. Hoppe') of people such as Mary
Pickford, Marlene  Dietrich, Albert Einstein, A.A. Milne, etc.  Apparently
E.O. Hoppe' (1878-1972) emigrated from Germany to Britain in 1900, was
awarded a fellowship in the Royal Photographic Society in 1907, and set up
a studio in London where he became the photographer of the famous and
beautiful; his subjects included the British royalty, Sarah Bernhardt,
Enrico Caruso, Mussolini, Coolidge, and anybody who was anybody.  Twain is
not specifically mentioned in the liner notes, but it would certainly seem
that Hoppe' is the guy you are looking for.  The notes state that further
information can be obtained (at least regarding the photographs in the CD
cases, and presumably others) by contacting:

E.O. Hoppe' Trust
c/o Curatorial Assistance, Inc
113 E. Union St.
Pasadena CA 91103-3927
phone 213-681-2401
fax 818-449-9603

I hope this turns out to be of help.  Let us know what turns up.
        Patrick Ober
        Wake Forest University