How do I add my ebooks to ~Books~ on my iPad

I have finally and reluctantly upgraded to iTunes 12.9.5.7 on my Windows 10 (1903) desktop.

The first thing I noted was that all the epub eBooks I had in "Books" have disappeared.

OK, I'm anal enough to have back ups.

Google this issue and about thirty sites tell me to drag and drop them from Windows Explorer into the "Books" folder(?) under "On My Device" on the iPad "Page".

BUT - I get a circle with a bar through it (the general prohibition sign).

I've tried adding files to the Library (individually or as a Folder) and after a short blink - nothing changes.

There are any number of alternative readers in the iTunes store that I can probably load my books into using "File Sharing" but it would be great to iTunes do what it should - and used to.