This How-To article is pretty thorough about configuring Plex to use local embedded metadata:
[HowTo] Configure Plex to use embedded metadata (music)
This one talks about selecting multiple items to edit simultaneously:
[HowTo] Selecting multiple items for edit, playlist, etc.
The problem with mixing audiobooks in with music files is because the agents are different. The audiobook agent tries to match with Audible.com’s database, and the music agent tries to match with MusicBrainz. Plex urges you to keep movies and TV separate from music files for the same reason.
If you configure Plex to prefer local metadata, and your metadata is very accurate, you may get away with having books and music in one library, but I really don’t think you’ll be very happy with it.
Another issue is the setting to “Store track progress.” This setting will only be useful if your audiobooks are in single, huge files. If your books are in multiple files, as they probably are if you ripped them from CDs, Plex will remember where you were in a particular track, but it won’t remember which track you were on. And the “Store track progress” setting will not be helpful with music files. Here’s an example of what will happen:
Playlists/Albums advance through the next song
