AudioBookConverter is very promising.
VBR AAC bitrate is only settable in five integer steps - so conversion seems a little vague,
That's how most audio encoders work, in presets.
But the feature that distinguishes it from fre:ac is being able to create M4B without reencoding.
The fields Narrator, Series, and Book # are Apple specific and cause problems in other audio players.
For instance the Narrator field gets put in the Composer tag(!).
This might be needed, but feels very wrong.
What do you mean by "problem"?
I expect a player that plays M4B can cope with an iTunes standard file.
Never seen an issue with unsupported tags. Every player I've used just shows the ones it knows and ignores the rest.
You might try to correct tags after creation using Foobar or MP3tag.
There never was a perfect match between the ID3 tags in MP3 designed for music and how we use them for audiobooks. Few players even display any tags other than the Title anyway.
So as long as they play and show the right title, cover image and chapter breaks, the rest is not a big deal, and rarely visible.
You can make a report on the GitHub page.
