How to add an audio file to Podcast in Catalina?
Same issue. I use a lot of classes as Audiobooks. Catalina has caused lot of trouble. Can't add to an existing book in Books.app.
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I have gotten around it somewhat by using a tag editor - Metadatics from the Mac App Store! Use it to reorder and group all your files of a particular audiobook. The Album name is the name of the audiobook. Add other details like Album Artwork, etc. Save the files.
Then drag the entire lot into Books.app. The files appear as one Audiobook. Cant add a file to an audiobook already there, except, to repeat above process with the new file/lecture.