Apple Books app not showing imported audiobooks

All of my audiobooks, purchased via Audible, have been imported into Apple Books app. They all used to show under the "Audiobooks" option. Having upgraded to Sequoia, I have opened Books for the first time and none show. The underlying files are (as far as I can see) are there under the Library/containers..... subfolders.

If I try to reimport, the Apple Books dialog won't let me select the hidden folders beneath Library.

Looks like an upgrade bug to me.