If you have ever wondered why audiobooks come as M4B while music comes as MP3, this is the short version, with practical advice on which to use when you have the choice.
The one-paragraph answer
M4B is an MP4 audio file (AAC) with a flag that tells players it is an audiobook. It can carry chapter markers, cover art and a remembered position in a single file. MP3 is a universal audio format with no built-in chapters; an audiobook in MP3 is usually a folder of files, one per chapter. If your player supports M4B (Apple Books, most audiobook apps), M4B is the better container. If you need the audio to play anywhere at all, MP3 wins.
Feature by feature
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Chapters: M4B yes, in the file. MP3 only by splitting into files (or ID3 chapter frames that few players read).
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Resume position: M4B players remember it per file. MP3 players usually remember per file too, but a 60-file book is fiddly.
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Cover art and metadata: both, but M4B’s audiobook tags (author, narrator, series) are better supported by audiobook apps.
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Sound quality per megabyte: AAC in M4B is slightly more efficient than MP3 at the same bitrate; for speech at 64 kbps the difference is small.
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Compatibility: MP3 plays on everything made since 1998. M4B plays on Apple devices and most modern audiobook apps, not on every car stereo.
Converting between them
Free tools handle both directions: Audiobook Builder (Mac), MP3 to M4B converters, and ffmpeg on any platform. Going from MP3 files to one M4B is common; you concatenate the files and write chapter markers. Going from M4B to MP3 loses the chapter markers unless you split into files.
How this applies to AI-narrated books
Book2Speech is a listening app rather than a file generator: narration streams into the web player and the iPhone app with chapters, position and read-along text, so the container question does not come up while listening. A full-book file can be requested from the book page for those who want one.