Buyers of subscription books ordered from a publisher's agent who came knocking on their door--not from bookstores. The agents went door-to-door with a sales prospectus, a book with sample pages to give potential buyers a taste for the real thing. Buyers also would choose a binding from back strips pasted into the flyleaf, and the publisher would print the book only after a certain number had been sold.
The University of Virginia's Barrett collection holds two of the original prospectuses, including the only one in existence with the "mutilated cut" --an illustration defaced by some anonymous jokester in the engraver's plant. The prospectuses have the same green cloth binding of the New York first edition; samples of back strips in other bindings; the frontispiece of Huck; tables of contents and illustrations; and 73 pages of text "meant to whet but not appease the appetite," to use Walter Blair's description. ("Mark Twain and Huck Finn, Berkeley: U of C Press 1960, p. 363). Also in the prospectuses: two pages of advertising, the wording drawn from the circulars that had been used to attract agents to canvass the book.
Twain had complete control over Huck Finn, for he had turned his nephew Charles Webster into a publishing company in 1884, just as he was finishing the book. "I am Webster & Co., myself" he announced. He had pushed for the subscription method for his books after the American Publishing Co. used it for "Innocents Abroad," his first big commercial success, writes Merle Johnson in "A Bibliography of the Works of Mark Twain," Harper and Bros., 1935.