[KevinH]

Again, as I wrote before, I created an epub3 using Sigil. I added a cover image file and properly manifested it and properly identified it in the opf metadata. No iTunesArtWork file exists.

This cover nicely shows in the Finder on macOS, and when I e-mailed it to my iPad and stored it in Books, the Books library shelf shows the cover. On the iPad Books read my epub the first time it was added and it created the iTunesArtwork file copying my epub's specified cover image. The same image file is there under two names. One the original image name manifested properly in the opf, and the second a iTunesArtwork file created and added to the epub to make accessing that image again and again much faster.

I tested this from scratch with my own epub created in Sigil on my macOS machine, and transferred to my iPad and opened in Books.

If you are seeing something else, then I am not sure how or why. The iTunesArtwork file is created based on what you tell it the cover image is, the very first time you load it in Books. Removing that file hurts nothing, as it will be properly recreated by Books the next time. If yours is not being recreated, then the issue is probably in the original epub missing something in the opf.