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 &quot;Remember playback position.&quot; I also usually set the equalizer to &quot;Spoken Word&quot; and just so the audio book doesn't come up when you are listening to your music on shuffle I would also check &quot;skip while shuffling.&quot;
I have found that setting the &quot;Media kind&quot; 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.