Nice plugin, but I have a couple of suggestions.
1. The Wordbooker custom fields are visible as you haven't added an underscore to hide them. This is most annoying as it clogs up the Custom Fields meta box, when all the options are already available in the Wordbooker meta box. e.g.
wordbooker_extract should be
wordbooker_options should be
_wordbooker_options to hide them from the custom fields meta box menu.
2. Is there any way of controlling which users can have access to or see the Wordbooker options? i.e. to have only editors see options while authors and below don't see anything?