Isn't a Half Title redundant, when the Title page already states the full
half Title? See Wikipedia:
I know why books must have blank pages, but why not leave this page blank?
I instance The Happy lawyer. The front cover contains the full title The Happy Lawyer: Making a Good Life in the Law. Then the first Half title page states only 'The Happy Lawyer'. Just after p. xiii (The Foreword), but before p. 1, is another Half title pages that states only 'The Happy Lawyer'.
Another instance is Better Never to Have Been. The front cover contains the full title 'Better Never to Have Been: The Harm Of Coming Into Existence'. Then the Half title page states only 'BETTER NEVER TO HAVE BEEN'.