The MetadataRetriever class has no support for ©mvn (MovementName) and ©mvi (MovementIndex), which is a bit unfortunate specifically for audio books.

Use case:

Audio books are often released in series (e.g. Harry Potter 1 - 7). To reflect this, ©mvn is often used to store the series name and ©mvi for the part number. Since ©mvi can only store integer values, some audio books have part numbers like 2.1, sometimes there are also custom tags stored in the form ----:com.apple.itunes:PART=2.1 or in my specific case ----:com.pilabor.tone:PART=2.1, which would also be nice to have.

Another thing that is not supported are chapters (mp4 supports quicktime or nero chapters), but this might be for another feature request.

Proposed solution:
Integrate support for at least these two and in the best case scenario also support custom tags for different domain names and providers.

For a wide variety of sample files you can check out atldotnet.

Alternatives:
The only alternative is to write a binary parser for mp4 and extract the required information.