Ah, the Twain Name Game. It is, indeed, an intriguing topic, for our 
man seems to have been known by different names to different people: Sam 
or Sammy to Hannibal friends, to his mother and brother, and to mining 
chums; usually Clemens to Howells and Rogers (as they were Howells and 
Rogers to him, in the Victorian tradition of gentlemen friends using 
last names -- think Holmes and Watson); Mark to Twichell and other close 
friends;  Youth to Livy. And he signs his letters differently, according 
to the recipient.
     I kicked this around a bit in "The Shape of the River" book, just 
for the sheer fun of it. It's just as intriguing (to me, at least) to 
see the primary name different biographers choose to employ. Some go 
with Sam. Some with Mark. Some go with Clemens. Some with Twain. 
Symbolically, I think it says a great deal about the richness of the 
writer and the personality we find so fascinating, but I'm not sure 
what. Maybe it's like the shape of the river, as Capt. Bixby explains it 
to young Sam (not yet Mark). Each time you try to get a fix on him, 
Mark-Sam-Clemens-Twain goes fooling around on you, changing form and 
daring you to keep up with him.