Combining .mp3 into .m4a Audio

[mgriebling]

There is a setting to combine mp3 files into a single m4b audio book.
Is there a way of just combining mp3 files in one directory into an audio book.
I have multiple directories containing mp3 files and would like one book (m4b file) per directory.
It's somewhat tedious to do this one directory at a time but it would be nice to do a batch of directories and
produce a set of books automatically. Is this possible?