Actually, it is a saved conversion setting but it doesn't require reconverting.
Preferences ==> Conversion ==> Otput Options ==> MOBI Output (tab) ==> Personal Doc tag: set to [EBOK]
Bulk Edit Metadata ==> Remove stored conversion settings for the selected books
Then resend.
The book will have the cdetype set to EBOK.
The Quality Check plugin on the other hand embeds an ASIN, which forces it to show up as a Book regardless. No need for setting the cdetype.