I've been searching for a bit, but I guess I'm not using the right terms or correct keywords to find what I'm after. My apologies if this is something I'm not seeing because of poor search kung fu!!
In my Apple Books (iPad Pro, iOS 15,6) library, I have a trilogy of books (I think this one was purchased through the apple bookstore) that show as a "stack" of the covers.

(attachment1:library-01.png)
When I tap the "stack" it opens up and I see the books laid out with "Book 1" and "Book 2" and such.

(attachment2:library-02.png)
I have other books (iirc, the books below were pruchased from ebooks.com) that are themselves part of various series and yet they are not "stacked" like this.

(attachment3:library-03.png)
I have no problem editing the metadata using Sigil or other similar programs, updating epub2 to epub3, and so on, but I can't seem to figure out
what to edit/add/etc to achieve the desired result (which is already there for a few series in my library).
Any guidance on how to replicate this?