Fieldname:true will find books with something in the field. Fieldname:false will find books with nothing in the field. For example, cover:true will find books with covers, and cover:false will find books without covers. Tags:true finds books with tags. Tags:#>3 finds books with more than three tags. And so on.
See
http://manual.calibre-ebook.com/gui....arch-interface for more information.