Not exclusively. Most of my audiobooks are ripped from CD, and the files aren’t always ‘per chapter.’ Occasionally it’s one file/chapter. More often it’s several files/chapter, and in some cases the files aren’t related to chapter breaks at all. In one case that I can think of, each file is roughly 3 minutes in length, and the chapter may change mid-file (what were they thinking?).
I’m fully in favor of local metadata support, whether there’s an online source or not. For me, it’s just like music. My books are fully tagged (embedded). I would much prefer local tags, whether they are embedded or in sidecar files. If it’s easier to omit online support altogether, that would have little or no effect on me.
The ID3 tags defined for .mp3 music should be easily usable:
- Composer (TCOM) = Author
- AlbumArtist (TPE2)* = Author
- TrackArtist (TPE1) = Narrator
- Grouping (TIT1) = Series
- Label (TPUB) = Publisher
*As I’ve noted WRT music, the ID3 spec doesn’t really have an “Album Artist” tag. Most apps, including Plex, have re-purposed the TPE2 tag. Why fight it?
The above are, of course, in addition to tags like disc #, track #, Sort Order fields, genre, etc.
There should be support for books written by multiple authors, as well as books read by multiple narrators. This, of course, is a very old failing for Music libraries. Now’s your chance to fix that. 
