About the Custom Fields Extension
The MBM Custom Fields Extension allows you to create your own fields to be used on your book pages. You can create fields of various types, and even create custom taxonomies to organize your books.
MBM Custom Fields integrates well with other MBM Extensions as well. When you create a custom taxonomy, it will be added into the book grid selection options just like the standard taxonomies like genre. It's added to both the regular book grid settings as well as the ones included with MBM Advanced Grids.
Additionally when combined with MBM Advanced Grids, you can use your custom fields as additional fields on your grid, and you can filter by your custom taxonomies. If you have Advanced Widgets installed, you can filter your widgets by custom taxonomies as well.