There’s a great article on the origins of book chapters by Nicholas Dames at The New Yorker:

Novels have always been good at absorbing and recycling, taking plots and devices from other genres and finding new uses for them. With the chapter, novelists began, in the eighteenth century, to naturalize an informational technology from antiquity by giving it a new cultural role. What the chapter did for the novel was to aerate it: by encouraging us to pause, stop, and put the book down–a chapter before bed, say–the chapter-break helps to root novels in the routines of everyday life. The chapter openly permitted a reading oriented around pauses–for reflection or rumination, perhaps, but also for refreshment or diversion. Laurence Sterne’s “Tristram Shandy” insisted that “chapters relieve the mind,” encouraging our immersion by letting us know that we will soon be allowed to exit and return to other tasks or demands. Coming and going–an attention paid out rhythmically–would become part of how novelists imagined their books would be read.
The formatting within the medium is the message?