The Lending Library as a Business
Before public libraries, reading was supplied commercially. A shop lent novels by the volume for a small charge, and the charge was small because the same copy earned it many times over. The economics were those of a rental business rather than a bookshop, and they shaped what was printed as much as what was read.
A novel issued in three volumes could be lent to three subscribers at once, and it cost three times as much to buy outright, which put purchase beyond most readers and kept them renting. Publishers obliged by producing three volumes whether or not the story required them, and authors wrote to fill the space. A form that looks like an artistic convention was in this case a commercial one, maintained by the largest customer that publishers had.
The arrangement ended quickly when the biggest circulating library refused to carry the format, and cheap single volumes replaced it within about two years. What is striking is not the collapse but how little the reading public had to do with it. The change came from a dispute between two businesses, and the shape of the novel changed as a by-product.