WordNet records no synonym and no antonym for letters. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the literary culture
“this book shows American letters at its best”
scholarly attainment
“he is a man of letters”
Synonyms, antonyms and the broader and narrower relations for letters come from Princeton University’s WordNet. Full sources and licences.