Stories existing in the same universe would have happened with or without capitalism. Sequel books are simply a way for authors to expound upon the events in their universe that they may have felt weren't concluded in the original/needed more than one book to be completed. Think of Homer's Iliad and Odyssey—two separate works that share major characters in the same general time frame, but that contain different adventures and thematic frameworks entirely. These stories predate capitalism by at least 2,000 years.

The caveat I guess would be book series that drag wearily on beyond their creative, natural conclusion for the sake of profit wouldn't exist without capitalism, obviously.