Harry Potter books missing from iPad

I recently discovered that the original Harry Potter ebooks, which I purchase from Pottermore when they were first available, no longer show up on my iPad, which is the device I use most frequently to read books. They appear in the Books app on my Mac and on my iPhone, but not the iPad. I don't know if this started when a separate iOS for the iPad was introduced, but that could make sense.

If I follow the online instruction for syncing books from my Mac to device -- connect device to Mac, select device from Finder sidebar, select Books from Button bar, choose the books to sync -- those books do NOT appear on the list of books on the Mac, even though they are on the compuer and accessible from the Mac Books app.

Anyone else have this problem? Any thoughts on how to fix it?