Is there a maintained package I can use to retrieve and set MP3 ID3 metadata using Python?
After some initial research I thought songdetails might fit my use case, but it doesn't handle .m4b files.  Mutagen does.  Note that while some have (reasonably) taken issue with Mutagen's surfacing of format-native keys, that vary from format to format (TIT2 for mp3, title for ogg, \xa9nam for mp4, Title for WMA etc.), mutagen.File() has a (new?) easy=True parameter that provides EasyMP3/EasyID3 tags, which have a consistent, albeit limited, set of keys.  I've only done limited testing so far, but the common keys, like album, artist, albumartist, genre, tracknumber, discnumber, etc. are all present and identical for .mb4 and .mp3 files when using easy=True, making it very convenient for my purposes.
Accessing MP3 metadata with Python