Splitting a book into multiple books can be complex.
You need to haul all the supporting pieces along.
EPUB split, a specialized tool does that.
This may appear simple for books with a single stylesheet and 1 or 2 images.
But take an anthology with graphic chapter numbers (1 per chapter), and Not even the same used in the next story

(Let us just do a EPUB Merge and slap on a new cover...

kind of book)
Befort that tool, I made (renamed) copies and tediously deleted the bits I did not want (and prayed I got it right

)