Aubrey Gordon collects vintage diet books. She has amassed almost 100 titles, including the 1973 volume “Slimming Down,” written by Johnny Carson’s sidekick, Ed McMahon. “Slimming Down” — which featured chapter titles like “The Breadstick Conspiracy” and “Two Martinis Into Connecticut” — is the book that began Ms. Gordon’s collection.And while the idea of mixology…