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MusicBrainz Picard will search for the correct metadata on its database, and even add a cover if there's one available. OP mentioned not being able to edit metadata on iTunes (I guess they were thinking of the Music app) you can use other metadata handlers if that is the case.
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This change was introduced when Apple phased out iTunes and made individual apps for each component: tv for movies and the subscription service, Music for the subscription service and your personal library they moved audiobooks to the Book app and Podcasts were stand alone already. If you do –add a cover, author, etc.– it will show up there and make the book look nicer.
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It's as simple as that, no need to modify metadata. You have to add them (drag them from the Finder if you want) to the Apple Books app.
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