'Category' is the genre of the book, not a collection. This option is useful if you want to group all books of a series together in the same category. One of the grouping options in iBooks' list display is to display books by category.
With respect to your plugboard error, confirm that the lookup name of your custom column is in fact 'read'. The lookup name is not the display name.
Last edited by GRiker; 01-13-2013 at 03:13 PM.