See the letter from SLC to Bliss, 2 Dec 1867, in Mark Twain's Letters, 
Volume
2, available online at marktwainproject.org:

"But I know Richardson, & learned from him, some months ago, something 
of an
idea of the subscription plan of publishing."

[Note:] Albert Deane Richardson (1833–69), a journalist and western 
traveler,
became chief war correspondent for the New York Tribune in 1860. 
Captured at
Vicksburg while attempting to run past the Confederate batteries with 
two other
reporters, he escaped from a Southern prison eighteen months later. 
After the
war he compiled two books incorporating his Tribune dispatches: The Secret
Service, the Field, the Dungeon, and the Escape (1865), which had a sale 
of one
hundred thousand copies, and Beyond the Mississippi (1867), which sold
seventy-five thousand copies by late 1869. Both books were published and 
sold by
the American Publishing Company (“Albert D. Richardson,” New York 
Tribune, 3
Dec 69, 1). Clemens’s conversation with Richardson “some months ago” 
probably
occurred in January 1867, for on 2 February he said in his letter to the 
Alta
that “Richardson is hard at work on his new book concerning the Far West,”
published in mid-1867 as Beyond the Mississippi. Richardson’s current 
project
for the American Publishing Company, which would bring him to 
Washington, was A
Personal History of Ulysses S. Grant (1868).

Harriet Smith