How to listen to old MP3 books on iPhone?

The first thing you want to do is get your MP3 files into iTunes. So copy those mp3 files to the Mac/PC you sync your iPhone with using iTunes. And drag them into the iTunes window to add them. Once there you can set the author, title and any other metadata you want to set. On a Mac click on the mp3 you want to edit. ⌘ Command i will bring up the dialog to do that.

One of the things to remember to do when setting the Author, title etc. is to click the options tab and check "Remember playback position." I also usually set the equalizer to "Spoken Word" and just so the audio book doesn't come up when you are listening to your music on shuffle I would also check "skip while shuffling."

I have found that setting the "Media kind" to anything but music often (at least for me) results in strange things happening, so just let iTunes think it is music.

Usually that will do it. It is not as good as a dedicated audio book player but it does work reasonably well.

iTunes turned out to be a dead end. It seems they have intentionally degraded iTunes so that one cannot set a .mp3 file to be an audiobook, so old books you already own must have a new license purchased. ($$$)

The solution was an iPhone app: "MP3 Audiobook Player"

Install it, plug your iPhone into your laptop, open then the Files tab, and drop the folder into "MP3 Audiobook Player" and Bingo! your old audiobook.

You have to add them (drag them from the Finder if you want) to the Apple Books app.

It's as simple as that, no need to modify metadata. If you do –add a cover, author, etc.– it will show up there and make the book look nicer.

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.

I'm on macOS Catalina 10.15.7 and Books app 2.4 (1923.1)

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. MusicBrainz Picard will search for the correct metadata on its database, and even add a cover if there's one available.

You can then sync the books and more with the Finder, look for your phone on the left and click on it. Then go to the "Audiobooks" section and check the proper options.

If you want to go wireless, you can use the free 5 GB tier if your book library is not too big.

To have the books synced to your iPhone you need to activate iCloud Drive.

and check "Books" on the Options. You don't need to activate any other app or service if this is the only thing you need synced.

In order to avoid many headaches, deactivate the Desktop and Document folders option, unless you know what you're doing (here's Apple Support guide in case you want to go that route).

You can sync bookmarks, highlights, and collections if you want; that's on the Books app preferences.