Almost anything will play the audio. The question is whether it understands the file as a book: chapters, position, speed, sleep timer. Here is what works.
iPhone and iPad
Apple Books is already installed and is the path of least resistance. AirDrop an M4B and choose Books. You get a chapter list, playback speed, a sleep timer, position synced across your devices through iCloud, and CarPlay controls that skip by chapter.
BookPlayer is the better app if you have a lot of books. Open source, folder-based, more control over speed and silence trimming, and it handles chapter marks properly.
Bound is worth a look too, particularly if you like a shelf-style library.
Android
There is no built-in audiobook app, and the default music player is a poor substitute — it plays the file and then forgets where you were. Install one of these first.
Smart AudioBook Player is the usual recommendation. Reads M4B chapters, keeps position per book, has a sleep timer with shake-to-extend, and adjusts speed sensibly.
Voice is open source, has no adverts, and does the basics well.
Listen has a cleaner interface with much the same capability.
Desktop
VLC plays anything and shows chapters in a menu, but it is not really a book app — it will not keep your place across sessions in a useful way.
Foobar2000 on Windows handles M4B chapters well and is endlessly configurable.
On the Mac, Apple Books is the obvious choice, and everything syncs with the phone.
Media servers
Audiobookshelf is purpose-built for this and worth setting up if you have more than a handful of books. It reads M4B chapters and cover art, tracks progress per user, and has good apps on both platforms.
Plex handles audiobooks reasonably through its music library, though it was not designed for them and it shows.
Jellyfin works, with the same caveat.
For all three, drop the file into the library folder and rescan.
In the car
CarPlay and Android Auto both surface chapter lists from an M4B, and the steering wheel controls skip by chapter. This is where chapter marks stop being a nicety.
A stereo reading a USB stick directly is a different matter and will usually refuse M4B. That is the one case where a folder of MP3s is genuinely the better choice.
What to check in any player
- Does skipping move by chapter, or by a fixed number of seconds?
- Does it still know your position after you force-quit it?
- Can it trim silence and change speed without pitching the voice up?
If a player fails the second one, it is treating your audiobook as music, and no file format will fix that.