lettering vs written material

lettering and written material both are nouns.

Word	Noun	Adjective	Verb	Adverb
lettering	Yes	No	No	No
written material	Yes	No	No	No

As nouns, written material is a hypernym of lettering; that is, written material is a word with a broader meaning than lettering:

lettering: letters inscribed (especially words engraved or carved) on something
written material: the work of a writer; anything expressed in letters of the alphabet (especially when considered from the point of view of style and effect)

Other hypernyms of lettering include piece of writing, writing.

lettering (noun)	written material (noun)
letters inscribed (especially words engraved or carved) on something	the work of a writer; anything expressed in letters of the alphabet (especially when considered from the point of view of style and effect)