How to edit metadata for Audiobooks in the Books app
I tried to answer this in the other thread but the reply button was greyed out so most likely the discussion was too old. Since macOS Catalina apparently, you can no longer edit the metadata in the Books app for audiobooks you (or someone you know and love like significant others) create. At least, if Apple has added the feature back I haven't found it.
So, what to do? Download Audacity - it's open source and free. OPEN or IMPORT your audio file. Once it's opened, go to the Edit menu and select Metadata. Edit your metadata there. Then EXPORT your file in whatever format available to you. I use mp3. If exporting to the same location, you'll either need to overwrite the existing file or give the file a new name.
You can also OPEN or IMPORT the file then EXPORT it without using the edit metadata feature. Simply give the file a new filename and the metadata window will open as part of the export process.
Once you've updated the metadata, import the file into Books. The updated metadata information should now appear.
It's a pain, I know. But it's the only work around I've been able to find. Perhaps Apple's developers will realize the mistake they made and give us back the ability to edit metadata from within the Books app but until then, this works.
MacBook Pro with Touch Bar