The Secret Letter is S
Categories:
Category 1: Name the TITLE of any work of Doctor Seuss
Category 2: Name a fantastic creature invented by Seuss WITH A RHYMING NAME
Category 3: Name an invented character by Seuss that is supposed to represent a human being.
Category 4: Name an anthropomorphic (like a human or having human features) device or creature by Seuss
Category 5: Name a real life person of consequence to Doctor Seuss (other than himself or a member of his nuclear family).
Category 6: Name a periodical publication that Seuss drew cartoons for.
Again, when it comes to a title, it doesn't need to START with an S. And S can be a first or last name.