I use MP3 Audiobook Player, but there are other similar apps available. MP3 Audiobook Player will play MP3 and M4B formats (perhaps others too, but these are the two I generally see). M4B with chapters is better than a single MP3 file; it’s easier to find your place again or relisten to a particular part. Once you’ve downloaded and unpacked the RAR or Zipfile or whatever to your computer, you then import the audiobook into MP3 Audiobook Player on your iPhone/iPad.
You can import from your computer via WiFi, and if you store audiobooks in the cloud, you can even import from iCloud Drive and other locations when you’re away from your computer.
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