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M4B — MPEG-4 Audiobook
M4B is the audiobook version of M4A: AAC audio in an MPEG-4 file with chapter marks and bookmarking, used by Apple Books and many audiobook apps.
An M4B file holds a whole audiobook in one file, with chapters and cover art, and players remember where you stopped. Apple Books, iTunes and many Android audiobook apps use it.
Car stereos, MP3 players and some apps do not play M4B. Converting to MP3 makes it play everywhere, though chapter marks are lost. Audiobooks bought with copy protection (DRM) cannot be converted.