How to keep the story straight:
Mark on the page which series (and if necessary, subseries) the book belongs to.
Add the book to a category; if there isn't one, make it with the official (double check your spelling!!) series name. Don't do subsets here, just main series/world.
On the category page, write out all the books you know of in the set, preferably in some kind of order.... Do please write out subsets here.
Voilà. You have just made the lives of staff easier.