Custom Taxonomy or Field With Options?

Think about how you want to use the field.  Do you want to be able to view a grid for each option?  Then you should use a taxonomy.

Do you know how many options a field will have? Is it a fixed set?  Then a field with options might be fine.  Do you want to be able to create new options right from the book page? Then you want a taxonomy.

Do you need to be able to hide the field due to potential spoilers?  Use a field with options.

Need the options listed in a certain order?  Use a field with options.