Create a Custom Taxonomy
Let’s create a custom taxonomy. Go to MBM Settings -> Custom Fields and click on the taxonomy field tab.
A taxonomy needs a slug. This forms the URL where you will be able to see all the books within that taxonomy (aka a taxonomy grid). Just like the standard taxonomies, MBM comes with, there are some restrictions. No WP reserved words, and no repeating other fields and taxonomies.
A default slug is set for you based on the singular name. You can customize it on the MBM Settings -> Book Grids page just like any other taxonomy.