I don't know what it's called but I refer to it by its suffix, a -logy or a set of compound works. From how the internet describes it it can be one singular story spanning over several novels or separate individual stories, but I like the first part of the definition when describing a series of books that has one core plot spanning over several consecutive novels. Compound Sets: 1.) Standalone 2.) Duology 3.) Trilogy 4.) Tetralogy or Quadrilogy (if you're Ridley Scott) 5.) Pentalogy 6.) Hexalogy 7.) Heptalogy 8.) Octalogy 9.) Ennealogy 10.) Decalogy 10+.) Polylogy (but I also read that this can be used to describe a set of two or more works, so this is more of an umbrella term) If any of this is wrong feel free to correct me.